ci: adjust build arguments (#2772)

The build arguments `VCS_REF` and `VCS_VER` were renamed and given
proper values according to their names.

1. `VCS_REVISION` holds the current SHA sum of the (git) HEAD pointer
2. `VCS_VERSION` now holds the contents of the `VERSION` file, i.e. a
   semver version tag (one can now inspect the image and find a proper
   version tag in the `org.opencontainers.image.version` label)

The build arguments were given defaults in order to allow the
`generic_build` and `generic_test` workflows to omit them (as they are
not need there anyways). When publishing images, this is fina as the
cache will rebuild almost all of the image except the last few layers
which are `LABEL`s anyways.

.github/workflows/generic_build.yml

@@ -83,9 +83,6 @@ jobs:
         uses: docker/build-push-action@v3.1.1
         with:
           context: .
-          build-args: |
-            VCS_REF=${{ github.sha }}
-            VCS_VER=${{ github.ref }}
           # Build at least the AMD64 image (which runs against the test suite).
           platforms: ${{ inputs.platforms }}
           # Paired with steps `actions/cache` and `Replace cache` (replace src with dest):

.github/workflows/generic_publish.yml

@@ -66,13 +66,18 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: 'Acquire the image version'
+        id: get-version
+        shell: bash
+        run: echo "::set-output name=version::$(<VERSION)"
+
       - name: 'Build and publish images'
         uses: docker/build-push-action@v3.1
         with:
           context: .
           build-args: |
-            VCS_REF=${{ github.sha }}
-            VCS_VER=${{ github.ref }}
+            VCS_REVISION=${{ github.sha }}
+            VCS_VERSION=${{ steps.get-version.outputs.version }}
           platforms: linux/amd64,linux/arm/v7,linux/arm64
           push: true
           tags: ${{ steps.prep.outputs.tags }}

.github/workflows/generic_test.yml

@@ -37,9 +37,6 @@ jobs:
         uses: docker/build-push-action@v3.1.1
         with:
           context: .
-          build-args: |
-            VCS_REF=${{ github.sha }}
-            VCS_VER=${{ github.ref }}
           tags: mailserver-testing:ci
           # Export the built image to the Docker host for use with BATS:
           load: true