62 lines
2.4 KiB
Markdown
62 lines
2.4 KiB
Markdown
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---
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stage: Verify
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group: Pipeline Execution
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/product/ux/technical-writing/#assignments
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type: concepts, howto
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---
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# Make Docker-in-Docker builds faster with Docker layer caching
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When using Docker-in-Docker, Docker downloads all layers of your image every
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time you create a build. Recent versions of Docker (Docker 1.13 and later) can
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use a pre-existing image as a cache during the `docker build` step. This significantly
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accelerates the build process.
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## How Docker caching works
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When running `docker build`, each command in `Dockerfile` creates a layer.
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These layers are retained as a cache and can be reused if there have been no changes. Change in one layer causes the recreation of all subsequent layers.
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To specify a tagged image to be used as a cache source for the `docker build`
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command, use the `--cache-from` argument. Multiple images can be specified
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as a cache source by using multiple `--cache-from` arguments. Any image that's used
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with the `--cache-from` argument must be pulled
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(using `docker pull`) before it can be used as a cache source.
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## Docker caching example
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This example `.gitlab-ci.yml` file shows how to use Docker caching:
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```yaml
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image: docker:20.10.16
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services:
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- docker:20.10.16-dind
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variables:
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# Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
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DOCKER_HOST: tcp://docker:2376
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DOCKER_TLS_CERTDIR: "/certs"
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before_script:
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- docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
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build:
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stage: build
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script:
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- docker pull $CI_REGISTRY_IMAGE:latest || true
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- docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $CI_REGISTRY_IMAGE:latest .
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- docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
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- docker push $CI_REGISTRY_IMAGE:latest
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```
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In the `script` section for the `build` stage:
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1. The first command tries to pull the image from the registry so that it can be
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used as a cache for the `docker build` command.
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1. The second command builds a Docker image by using the pulled image as a
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cache (see the `--cache-from $CI_REGISTRY_IMAGE:latest` argument) if
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available, and tags it.
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1. The last two commands push the tagged Docker images to the container registry
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so that they can also be used as cache for subsequent builds.
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