ci: reuse docker images across jobs

We were rebuilding Docker images from scratch with --no-cache, tagged by PR
number. With this, we were building multiple images per PR for the same
context.

This change tags the images by the git tree hash of the build context and only
build if the tag is missing. This naturally covers rebuilding on build context
changes.

Finally, this change handles refreshing images every 7 days (for example to
refresh OS packages) and prune by CI label after 14 days. The gap ensures an
image a running job just picked up isn't pruned under it.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

.github/workflows/yocto-layer.yml

@@ -22,24 +22,22 @@ jobs:
       - name: Check for Yocto relevant changes
         id: changed-files-specific
         uses: ./.github/actions/changed-files
-      - name: Build a temporary yocto-builder image
+      - name: Build the yocto-builder image
+        id: docker
         uses: ./.github/actions/docker-build
         with:
           docker_image: yocto-builder
-          id: ${{ github.event.number }}
         if: steps.changed-files-specific.outputs.any_changed == 'true'
       - name: Run yocto-check-layer
         run: |
           docker run --rm --security-opt apparmor=unconfined \
             -v "$GITHUB_WORKSPACE:/work:ro" \
             --env "BASE_REF=$GITHUB_BASE_REF" \
-            "yocto-builder-${{ github.event.number }}" \
+            "${{ steps.docker.outputs.image }}" \
             /entrypoint-yocto-check-layer.sh
         if: steps.changed-files-specific.outputs.any_changed == 'true'
-      - name: Cleanup temporary docker image
-        uses: ./.github/actions/docker-clean-image
-        with:
-          docker_image: yocto-builder-${{ github.event.number }}
+      - name: Prune old CI docker images
+        uses: ./.github/actions/docker-prune-images
         if: always()
       - name: Cleanup dangling docker images
         uses: ./.github/actions/docker-clean-dangling