debian-mirror-gitlab/doc/ci/docker/using_docker_build.md

---
stage: Verify
group: Continuous Integration
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
type: concepts, howto
---

# Building Docker images with GitLab CI/CD

You can use GitLab CI/CD with Docker Engine to build and test Docker-based projects.

For example, you might want to:

1. Create an application image.
1. Run tests against the created image.
1. Push image to a remote registry.
1. Deploy to a server from the pushed image.

Or, if your application already has a `Dockerfile`, you can
use it to create and test an image:

```shell
docker build -t my-image dockerfiles/
docker run my-image /script/to/run/tests
docker tag my-image my-registry:5000/my-image
docker push my-registry:5000/my-image
```

To run Docker commands in your CI/CD jobs, you must configure
GitLab Runner to enable `docker` support.

## Enable Docker commands in your CI/CD jobs

There are three ways to enable the use of `docker build` and `docker run`
during jobs, each with their own tradeoffs. You can use:

- [The shell executor](#use-the-shell-executor)
- [The Docker executor with the Docker image (Docker-in-Docker)](#use-the-docker-executor-with-the-docker-image-docker-in-docker)
- [Docker socket binding](#use-docker-socket-binding)

If you don't want to execute a runner in privileged mode,
but want to use `docker build`, you can also [use kaniko](using_kaniko.md).

If you are using shared runners on GitLab.com, see
[GitLab.com shared runners](../../user/gitlab_com/index.md#shared-runners)
to learn more about how these runners are configured.

### Use the shell executor

One way to configure GitLab Runner for `docker` support is to use the
`shell` executor.

After you register a runner and select the `shell` executor,
your job scripts are executed as the `gitlab-runner` user.
This user needs permission to run Docker commands.

1. [Install](https://gitlab.com/gitlab-org/gitlab-runner/#installation) GitLab Runner.
1. [Register](https://docs.gitlab.com/runner/register/) a runner.
   Select the `shell` executor. For example:

   ```shell
   sudo gitlab-runner register -n \
     --url https://gitlab.com/ \
     --registration-token REGISTRATION_TOKEN \
     --executor shell \
     --description "My Runner"
   ```

1. On the server where GitLab Runner is installed, install Docker Engine.
   View a list of [supported platforms](https://docs.docker.com/engine/installation/).

1. Add the `gitlab-runner` user to the `docker` group:

   ```shell
   sudo usermod -aG docker gitlab-runner
   ```

1. Verify that `gitlab-runner` has access to Docker:

   ```shell
   sudo -u gitlab-runner -H docker info
   ```

1. In GitLab, to verify that everything works, add `docker info` to `.gitlab-ci.yml`:

   ```yaml
   before_script:
     - docker info

   build_image:
     script:
       - docker build -t my-docker-image .
       - docker run my-docker-image /script/to/run/tests
   ```

You can now use `docker` commands (and install `docker-compose` if needed).

When you add `gitlab-runner` to the `docker` group, you are effectively granting `gitlab-runner` full root permissions.
Learn more about the [security of the `docker` group](https://blog.zopyx.com/on-docker-security-docker-group-considered-harmful/).

### Use the Docker executor with the Docker image (Docker-in-Docker)

Another way to configure GitLab Runner for `docker` support is to
register a runner with the Docker executor and use the [Docker image](https://hub.docker.com/_/docker/)
to run your job scripts. This configuration is referred to as "Docker-in-Docker."

The Docker image has all of the `docker` tools installed
and can run the job script in context of the image in privileged mode.

The `docker-compose` command is not available in this configuration by default.
To use `docker-compose` in your job scripts, follow the `docker-compose`
[installation instructions](https://docs.docker.com/compose/install/).

WARNING:
When you enable `--docker-privileged`, you are effectively disabling all of
the security mechanisms of containers and exposing your host to privilege
escalation which can lead to container breakout. For more information, check
out the official Docker documentation on
[runtime privilege and Linux capabilities](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities).

Docker-in-Docker works well, and is the recommended configuration, but it is
not without its own challenges:

- When using Docker-in-Docker, each job is in a clean environment without the past
  history. Concurrent jobs work fine because every build gets its own
  instance of Docker engine so they don't conflict with each other. But this
  also means that jobs can be slower because there's no caching of layers.
- By default, Docker 17.09 and higher uses `--storage-driver overlay2` which is
  the recommended storage driver. See [Using the overlayfs driver](#use-the-overlayfs-driver)
  for details.
- Since the `docker:19.03.12-dind` container and the runner container don't share their
  root file system, the job's working directory can be used as a mount point for
  child containers. For example, if you have files you want to share with a
  child container, you may create a subdirectory under `/builds/$CI_PROJECT_PATH`
  and use it as your mount point (for a more thorough explanation, check [issue
  #41227](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/41227)):

  ```yaml
  variables:
    MOUNT_POINT: /builds/$CI_PROJECT_PATH/mnt

  script:
    - mkdir -p "$MOUNT_POINT"
    - docker run -v "$MOUNT_POINT:/mnt" my-docker-image
  ```

An example project using this approach can be found here: <https://gitlab.com/gitlab-examples/docker>.

In the examples below, we are using Docker images tags to specify a
specific version, such as `docker:19.03.12`. If tags like `docker:stable`
are used, you have no control over what version is used. This can lead to
unpredictable behavior, especially when new versions are
released.

#### TLS enabled

The Docker daemon supports connection over TLS and it's done by default
for Docker 19.03.12 or higher. This is the **suggested** way to use the
Docker-in-Docker service and
[GitLab.com shared runners](../../user/gitlab_com/index.md#shared-runners)
support this.

##### Docker

> Introduced in GitLab Runner 11.11.

1. Install [GitLab Runner](https://docs.gitlab.com/runner/install/).
1. Register GitLab Runner from the command line to use `docker` and `privileged`
   mode:

   ```shell
   sudo gitlab-runner register -n \
     --url https://gitlab.com/ \
     --registration-token REGISTRATION_TOKEN \
     --executor docker \
     --description "My Docker Runner" \
     --docker-image "docker:19.03.12" \
     --docker-privileged \
     --docker-volumes "/certs/client"
   ```

   The above command registers a new runner to use the special
   `docker:19.03.12` image, which is provided by Docker. **Notice that it's
   using the `privileged` mode to start the build and service
   containers.** If you want to use [Docker-in-Docker](https://www.docker.com/blog/docker-can-now-run-within-docker/) mode, you always
   have to use `privileged = true` in your Docker containers.

   This also mounts `/certs/client` for the service and build
   container, which is needed for the Docker client to use the
   certificates inside of that directory. For more information on how
   Docker with TLS works, check <https://hub.docker.com/_/docker/#tls>.

   The above command creates a `config.toml` entry similar to this:

   ```toml
   [[runners]]
     url = "https://gitlab.com/"
     token = TOKEN
     executor = "docker"
     [runners.docker]
       tls_verify = false
       image = "docker:19.03.12"
       privileged = true
       disable_cache = false
       volumes = ["/certs/client", "/cache"]
     [runners.cache]
       [runners.cache.s3]
       [runners.cache.gcs]
   ```

1. You can now use `docker` in the build script (note the inclusion of the
   `docker:19.03.12-dind` service):

   ```yaml
   image: docker:19.03.12

   variables:
     # When using dind service, we need to instruct docker to talk with
     # the daemon started inside of the service. The daemon is available
     # with a network connection instead of the default
     # /var/run/docker.sock socket. Docker 19.03 does this automatically
     # by setting the DOCKER_HOST in
     # https://github.com/docker-library/docker/blob/d45051476babc297257df490d22cbd806f1b11e4/19.03/docker-entrypoint.sh#L23-L29
     #
     # The 'docker' hostname is the alias of the service container as described at
     # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
     #
     # Specify to Docker where to create the certificates, Docker will
     # create them automatically on boot, and will create
     # `/certs/client` that will be shared between the service and job
     # container, thanks to volume mount from config.toml
     DOCKER_TLS_CERTDIR: "/certs"

   services:
     - docker:19.03.12-dind

   before_script:
     - docker info

   build:
     stage: build
     script:
       - docker build -t my-docker-image .
       - docker run my-docker-image /script/to/run/tests
   ```

##### Kubernetes

> [Introduced](https://gitlab.com/gitlab-org/charts/gitlab-runner/-/issues/106) in GitLab Runner Helm Chart 0.23.0.

1. Using the
   [Helm chart](https://docs.gitlab.com/runner/install/kubernetes.html), update the
   [`values.yml` file](https://gitlab.com/gitlab-org/charts/gitlab-runner/-/blob/00c1a2098f303dffb910714752e9a981e119f5b5/values.yaml#L133-137)
   to specify a volume mount.

   ```yaml
   runners:
     config: |
       [[runners]]
         [runners.kubernetes]
           image = "ubuntu:20.04"
           privileged = true
         [[runners.kubernetes.volumes.empty_dir]]
           name = "docker-certs"
           mount_path = "/certs/client"
           medium = "Memory"
   ```

1. You can now use `docker` in the build script (note the inclusion of the
   `docker:19.03.13-dind` service):

   ```yaml
   image: docker:19.03.13

   variables:
     # When using dind service, we need to instruct docker to talk with
     # the daemon started inside of the service. The daemon is available
     # with a network connection instead of the default
     # /var/run/docker.sock socket.
     DOCKER_HOST: tcp://docker:2376
     #
     # The 'docker' hostname is the alias of the service container as described at
     # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
     # If you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
     # the variable must be set to tcp://localhost:2376 because of how the
     # Kubernetes executor connects services to the job container
     # DOCKER_HOST: tcp://localhost:2376
     #
     # Specify to Docker where to create the certificates, Docker will
     # create them automatically on boot, and will create
     # `/certs/client` that will be shared between the service and job
     # container, thanks to volume mount from config.toml
     DOCKER_TLS_CERTDIR: "/certs"
     # These are usually specified by the entrypoint, however the
     # Kubernetes executor doesn't run entrypoints
     # https://gitlab.com/gitlab-org/gitlab-runner/-/issues/4125
     DOCKER_TLS_VERIFY: 1
     DOCKER_CERT_PATH: "$DOCKER_TLS_CERTDIR/client"

   services:
     - docker:19.03.13-dind

   before_script:
     - docker info

   build:
     stage: build
     script:
       - docker build -t my-docker-image .
       - docker run my-docker-image /script/to/run/tests
   ```

#### TLS disabled

Sometimes there are legitimate reasons why you might want to disable TLS.
For example, you have no control over the GitLab Runner configuration
that you are using.

Assuming that the runner's `config.toml` is similar to:

```toml
[[runners]]
  url = "https://gitlab.com/"
  token = TOKEN
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:19.03.12"
    privileged = true
    disable_cache = false
    volumes = ["/cache"]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
```

You can now use `docker` in the build script (note the inclusion of the
`docker:19.03.12-dind` service):

```yaml
image: docker:19.03.12

variables:
  # When using dind service we need to instruct docker, to talk with the
  # daemon started inside of the service. The daemon is available with
  # a network connection instead of the default /var/run/docker.sock socket.
  #
  # The 'docker' hostname is the alias of the service container as described at
  # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services
  #
  # If you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
  # the variable must be set to tcp://localhost:2375 because of how the
  # Kubernetes executor connects services to the job container
  # DOCKER_HOST: tcp://localhost:2375
  #
  DOCKER_HOST: tcp://docker:2375
  #
  # This will instruct Docker not to start over TLS.
  DOCKER_TLS_CERTDIR: ""

services:
  - docker:19.03.12-dind

before_script:
  - docker info

build:
  stage: build
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests
```

### Use Docker socket binding

Another way to configure GitLab Runner for `docker` support is to
bind-mount `/var/run/docker.sock` into the
container so that Docker is available in the context of the image.

NOTE:
If you bind the Docker socket and you are
[using GitLab Runner 11.11 or later](https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1261),
you can no longer use `docker:19.03.12-dind` as a service. Volume bindings
are done to the services as well, making these incompatible.

To make Docker available in the context of the image:

1. Install [GitLab Runner](https://docs.gitlab.com/runner/install/).
1. From the command line, register a runner with the `docker` executor and share `/var/run/docker.sock`:

   ```shell
   sudo gitlab-runner register -n \
     --url https://gitlab.com/ \
     --registration-token REGISTRATION_TOKEN \
     --executor docker \
     --description "My Docker Runner" \
     --docker-image "docker:19.03.12" \
     --docker-volumes /var/run/docker.sock:/var/run/docker.sock
   ```

   This command registers a new runner to use the special
   `docker:19.03.12` image, which is provided by Docker. **The command uses
   the Docker daemon of the runner itself. Any containers spawned by Docker
   commands are siblings of the runner rather than children of the runner.**
   This may have complications and limitations that are unsuitable for your workflow.

   Your `config.toml` file should now have an entry like this:

   ```toml
   [[runners]]
     url = "https://gitlab.com/"
     token = REGISTRATION_TOKEN
     executor = "docker"
     [runners.docker]
       tls_verify = false
       image = "docker:19.03.12"
       privileged = false
       disable_cache = false
       volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
     [runners.cache]
       Insecure = false
   ```

1. Use `docker` in the build script. You don't need to
   include the `docker:19.03.12-dind` service, like you do when you're using
   the Docker-in-Docker executor:

   ```yaml
   image: docker:19.03.12

   before_script:
     - docker info

   build:
     stage: build
     script:
       - docker build -t my-docker-image .
       - docker run my-docker-image /script/to/run/tests
   ```

This method avoids using Docker in privileged mode. However,
the implications of this method are:

- By sharing the Docker daemon, you are effectively disabling all
  the security mechanisms of containers and exposing your host to privilege
  escalation, which can lead to container breakout. For example, if a project
  ran `docker rm -f $(docker ps -a -q)` it would remove the GitLab Runner
  containers.
- Concurrent jobs may not work; if your tests
  create containers with specific names, they may conflict with each other.
- Sharing files and directories from the source repository into containers may not
  work as expected. Volume mounting is done in the context of the host
  machine, not the build container. For example:

   ```shell
   docker run --rm -t -i -v $(pwd)/src:/home/app/src test-image:latest run_app_tests
   ```

#### Enable registry mirror for `docker:dind` service

When the Docker daemon starts inside of the service container, it uses
the default configuration. You may want to configure a [registry
mirror](https://docs.docker.com/registry/recipes/mirror/) for
performance improvements and ensuring you don't reach DockerHub rate limits.

##### Inside `.gitlab-ci.yml`

You can append extra CLI flags to the `dind` service to set the registry
mirror:

```yaml
services:
   - name: docker:19.03.13-dind
     command: ["--registry-mirror", "https://registry-mirror.example.com"] # Specify the registry mirror to use.
```

##### DinD service defined inside of GitLab Runner configuration

> [Introduced](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27173) in GitLab Runner 13.6.

If you are an administrator of GitLab Runner and you have the `dind`
service defined for the [Docker
executor](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersdockerservices-section),
or the [Kubernetes
executor](https://docs.gitlab.com/runner/executors/kubernetes.html#using-services)
you can specify the `command` to configure the registry mirror for the
Docker daemon.

Docker:

```toml
[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    ...
    privileged = true
    [[runners.docker.services]]
      name = "docker:19.03.13-dind"
      command = ["--registry-mirror", "https://registry-mirror.example.com"]
```

Kubernetes:

```toml
[[runners]]
  ...
  name = "kubernetes"
  [runners.kubernetes]
    ...
    privileged = true
    [[runners.kubernetes.services]]
      name = "docker:19.03.13-dind"
      command = ["--registry-mirror", "https://registry-mirror.example.com"]
```

##### Docker executor inside GitLab Runner configuration

If you are an administrator of GitLab Runner and you want to use
the mirror for every `dind` service, update the
[configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html)
to specify a [volume
mount](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#volumes-in-the-runnersdocker-section).

For example, if you have a `/opt/docker/daemon.json` file with the following
content:

```json
{
  "registry-mirrors": [
    "https://registry-mirror.example.com"
  ]
}
```

Update the `config.toml` file to mount the file to
`/etc/docker/daemon.json`. This would mount the file for **every**
container that is created by GitLab Runner. The configuration is
picked up by the `dind` service.

```toml
[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    image = "alpine:3.12"
    privileged = true
    volumes = ["/opt/docker/daemon.json:/etc/docker/daemon.json:ro"]
```

##### Kubernetes executor inside GitLab Runner configuration

> [Introduced](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/3223) in GitLab Runner 13.6.

If you are an administrator of GitLab Runner and you want to use
the mirror for every `dind` service, update the
[configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html)
to specify a [ConfigMap volume
mount](https://docs.gitlab.com/runner/executors/kubernetes.html#using-volumes).

For example, if you have a `/tmp/daemon.json` file with the following
content:

```json
{
  "registry-mirrors": [
    "https://registry-mirror.example.com"
  ]
}
```

Create a [ConfigMap](https://kubernetes.io/docs/concepts/configuration/configmap/) with the content
of this file. You can do this with a command like:

```shell
kubectl create configmap docker-daemon --namespace gitlab-runner --from-file /tmp/daemon.json
```

NOTE:
Make sure to use the namespace that GitLab Runner Kubernetes executor uses
to create job pods in.

After the ConfigMap is created, you can update the `config.toml`
file to mount the file to `/etc/docker/daemon.json`. This update
mounts the file for **every** container that is created by GitLab Runner.
The configuration is picked up by the `dind` service.

```toml
[[runners]]
  ...
  executor = "kubernetes"
  [runners.kubernetes]
    image = "alpine:3.12"
    privileged = true
    [[runners.kubernetes.volumes.config_map]]
      name = "docker-daemon"
      mount_path = "/etc/docker/daemon.json"
      sub_path = "daemon.json"
```

## Authenticating with registry in Docker-in-Docker

When you use Docker-in-Docker, the [normal authentication
methods](using_docker_images.html#define-an-image-from-a-private-container-registry)
won't work because a fresh Docker daemon is started with the service.

### Option 1: Run `docker login`

In [`before_script`](../yaml/README.md#before_script) run `docker
login`:

```yaml
image: docker:19.03.13

variables:
  DOCKER_TLS_CERTDIR: "/certs"

services:
  - docker:19.03.13-dind

build:
  stage: build
  before_script:
    - echo "$DOCKER_REGISTRY_PASS" | docker login $DOCKER_REGISTRY --username $DOCKER_REGISTRY_USER --password-stdin
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests
```

To log in to Docker Hub, leave `$DOCKER_REGISTRY`
empty or remove it.

### Option 2: Mount `~/.docker/config.json` on each job

If you are an administrator for GitLab Runner, you can mount a file
with the authentication configuration to `~/.docker/config.json`.
Then every job that the runner picks up will be authenticated already. If you
are using the official `docker:19.03.13` image, the home directory is
under `/root`.

If you mount the configuration file, any `docker` command
that modifies the `~/.docker/config.json` (for example, `docker login`)
fails, because the file is mounted as read-only. Do not change it from
read-only, because other problems will occur.

Here is an example of `/opt/.docker/config.json` that follows the
[`DOCKER_AUTH_CONFIG`](using_docker_images.md#determining-your-docker_auth_config-data)
documentation:

```json
{
    "auths": {
        "https://index.docker.io/v1/": {
            "auth": "bXlfdXNlcm5hbWU6bXlfcGFzc3dvcmQ="
        }
    }
}
```

#### Docker

Update the [volume
mounts](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#volumes-in-the-runnersdocker-section)
to include the file.

```toml
[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    ...
    privileged = true
    volumes = ["/opt/.docker/config.json:/root/.docker/config.json:ro"]
```

#### Kubernetes

Create a [ConfigMap](https://kubernetes.io/docs/concepts/configuration/configmap/) with the content
of this file. You can do this with a command like:

```shell
kubectl create configmap docker-client-config --namespace gitlab-runner --from-file /opt/.docker/config.json
```

Update the [volume
mounts](https://docs.gitlab.com/runner/executors/kubernetes.html#using-volumes)
to include the file.

```toml
[[runners]]
  ...
  executor = "kubernetes"
  [runners.kubernetes]
    image = "alpine:3.12"
    privileged = true
    [[runners.kubernetes.volumes.config_map]]
      name = "docker-client-config"
      mount_path = "/root/.docker/config.json"
      # If you are running GitLab Runner 13.5
      # or lower you can remove this
      sub_path = "config.json"
```

### Option 3: Use `DOCKER_AUTH_CONFIG`

If you already have
[`DOCKER_AUTH_CONFIG`](using_docker_images.md#determining-your-docker_auth_config-data)
defined, you can use the variable and save it in
`~/.docker/config.json`.

There are multiple ways to define this. For example:

- Inside
  [`pre_build_script`](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section)
  inside of the runner configuration file.
- Inside [`before_script`](../yaml/README.md#before_script).
- Inside of [`script`](../yaml/README.md#script).

Below is an example of
[`before_script`](../yaml/README.md#before_script). The same commands
apply for any solution you implement.

```yaml
image: docker:19.03.13

variables:
  DOCKER_TLS_CERTDIR: "/certs"

services:
  - docker:19.03.13-dind

build:
  stage: build
  before_script:
    - mkdir -p $HOME/.docker
    - echo $DOCKER_AUTH_CONFIG > $HOME/.docker/config.json
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests
```

## Making Docker-in-Docker builds faster with Docker layer caching

When using Docker-in-Docker, Docker downloads all layers of your image every
time you create a build. Recent versions of Docker (Docker 1.13 and above) can
use a pre-existing image as a cache during the `docker build` step, considerably
speeding up the build process.

### How Docker caching works

When running `docker build`, each command in `Dockerfile` results in a layer.
These layers are kept around as a cache and can be reused if there haven't been
any changes. Change in one layer causes all subsequent layers to be recreated.

You can specify a tagged image to be used as a cache source for the `docker build`
command by using the `--cache-from` argument. Multiple images can be specified
as a cache source by using multiple `--cache-from` arguments. Keep in mind that
any image that's used with the `--cache-from` argument must first be pulled
(using `docker pull`) before it can be used as a cache source.

### Using Docker caching

Here's a `.gitlab-ci.yml` file showing how Docker caching can be used:

```yaml
image: docker:19.03.12

services:
  - docker:19.03.12-dind

variables:
  # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
  DOCKER_HOST: tcp://docker:2376
  DOCKER_TLS_CERTDIR: "/certs"

before_script:
  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY

build:
  stage: build
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker push $CI_REGISTRY_IMAGE:latest
```

The steps in the `script` section for the `build` stage can be summed up to:

1. The first command tries to pull the image from the registry so that it can be
   used as a cache for the `docker build` command.
1. The second command builds a Docker image using the pulled image as a
   cache (notice the `--cache-from $CI_REGISTRY_IMAGE:latest` argument) if
   available, and tags it.
1. The last two commands push the tagged Docker images to the container registry
   so that they may also be used as cache for subsequent builds.

## Use the OverlayFS driver

NOTE:
The shared runners on GitLab.com use the `overlay2` driver by default.

By default, when using `docker:dind`, Docker uses the `vfs` storage driver which
copies the filesystem on every run. This is a disk-intensive operation
which can be avoided if a different driver is used, for example `overlay2`.

### Requirements

1. Make sure a recent kernel is used, preferably `>= 4.2`.
1. Check whether the `overlay` module is loaded:

   ```shell
   sudo lsmod | grep overlay
   ```

   If you see no result, then it isn't loaded. To load it use:

   ```shell
   sudo modprobe overlay
   ```

   If everything went fine, you need to make sure module is loaded on reboot.
   On Ubuntu systems, this is done by editing `/etc/modules`. Just add the
   following line into it:

   ```plaintext
   overlay
   ```

### Use the OverlayFS driver per project

You can enable the driver for each project individually by using the `DOCKER_DRIVER`
environment [variable](../yaml/README.md#variables) in `.gitlab-ci.yml`:

```yaml
variables:
  DOCKER_DRIVER: overlay2
```

### Use the OverlayFS driver for every project

If you use your own [GitLab Runners](https://docs.gitlab.com/runner/), you
can enable the driver for every project by setting the `DOCKER_DRIVER`
environment variable in the
[`[[runners]]` section of `config.toml`](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section):

```toml
environment = ["DOCKER_DRIVER=overlay2"]
```

If you're running multiple runners, you have to modify all configuration files.

Read more about the [runner configuration](https://docs.gitlab.com/runner/configuration/)
and [using the OverlayFS storage driver](https://docs.docker.com/engine/userguide/storagedriver/overlayfs-driver/).

## Using the GitLab Container Registry

After you've built a Docker image, you can push it up to the built-in
[GitLab Container Registry](../../user/packages/container_registry/index.md#build-and-push-by-using-gitlab-cicd).

## Troubleshooting

### `docker: Cannot connect to the Docker daemon at tcp://docker:2375. Is the docker daemon running?`

This is a common error when you are using
[Docker in Docker](#use-the-docker-executor-with-the-docker-image-docker-in-docker)
v19.03 or higher.

This occurs because Docker starts on TLS automatically, so you need to do some setup.
If:

- This is the first time setting it up, carefully read
  [using Docker in Docker workflow](#use-the-docker-executor-with-the-docker-image-docker-in-docker).
- You are upgrading from v18.09 or earlier, read our
  [upgrade guide](https://about.gitlab.com/releases/2019/07/31/docker-in-docker-with-docker-19-dot-03/).