This action is designed to perform multistage Docker builds in a straightforward and fast way. As it turns out, this is surprisingly difficult to do well in CI, since the host machine performing the build typically starts in a clean slate each time, which means most of the layer caching used by Docker becomes moot. Trying to use Github's actions/cache to work around this can be quite challenging, and manually dealing with each stage in the build requires a lot of repetition in the Action YAML.

The inputs to this action allow you to specify the various build stage names as cache targets that will be created and pushed to the registry for future re-use. Each stage will be tagged using the branch name and full commit hash. While the initial build will, of course, be performed from scratch, subsequent builds will pull the previously-built images that the layer caching can use.

The following Actions workflow file will:

• Check out the code
• Authenticate to the Docker registry
• Perform the multistage build (pulling previous images as needed)
• Run the test image
• Deploy the server if tests pass

on:
  push:
    branches:
      - main
  pull_request:

jobs:
  build-and-test:
    name: Build and test
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2

      # This step MUST be performed before multistage-docker-build
      - name: Auth to GH registry
        uses: docker/login-action@v1
        with:
          registry: ghcr.io
          username: ${{ github.repository_owner }}
          password: ${{ secrets.GITHUB_TOKEN }}

      - uses: firehed/multistage-docker-build-action@v1
        id: build
        with:
          dockerfile: examples/Dockerfile
          repository: ghcr.io/firehed/actions
          stages: env, configured
          testenv-stage: testenv
          server-stage: server
          build-args: arg1=val1, arg2=val2

      # This assumes your testenv actually runs the tests, and
      # exits 0 if they all pass and nonzero on failure
      - name: Run tests
        run: docker run ${{ steps.build.outputs.testenv-tag }}
        
      # This can be any command, and you will probably need to
      # do additional setup first
      - name: Deploy
        run: kubectl set image deploy myapp server=${{ steps.build.outputs.server-tag }}

The following images will exist:

• ghcr.io/firehed/actions/server:{commit-hash}
• ghcr.io/firehed/actions/server:latest (if the action runs on the default branch, e.g. "main" or "master")
• ghcr.io/firehed/actions/testenv:{commit-hash} (if testenv-stage is provided)

The intended use-case is that the testenv will be used for further testing in CI, and the server will eventually be deployed. You may want remove the intermediate branch images when the branch is closed to save on storage.

The following images will also be created, but they are for internal use only (relating to layer caching). You should not use, deploy, or otherwise depend on them - they may change at any time!

• ghcr.io/firehed/actions/env:{branch-related-name}
• ghcr.io/firehed/actions/configured:{branch-related-name}

tl:dr: If it comes from one of the outputs of this action, go ahead and use it. If not, don't!

• Use with Docker Buildkit (via DOCKER_BUILDKIT=1) does not consistently use the layer caches. This seems to be a Buildkit issue. It's recommended to leave Buildkit disabled at this time.
• Make a straightforward mechanism to do cleanup