chore: pin rust container version

.woodpecker.yml

@@ -1,6 +1,6 @@
-pipeline:
+steps:
   backend:
-    image: rust
+    image: rust:1.79-bullseye
     environment:
       - DATABASE_URL=postgres://postgres:password@database:5432/postgres
     commands:
@@ -20,6 +20,7 @@ pipeline:
     image: plugins/docker
     when:
       event: [push, tag, deployment]
+      branch: master
     settings:
       username: realaravinth
       password: