ci: add Docker buildx GHA layer cache to build workflows

Caches Docker build layers in GitHub Actions cache (type=gha) scoped
per platform. The second build-push-action call in the update workflow
(push step) now reuses layers warmed by the test-image step, making
the push build effectively free for this scratch-based image and
future-proofing for any Dockerfile growth.

Author: chorrell

.github/workflows/dockerimage.yml

@@ -77,6 +77,8 @@ jobs:
           platforms: linux/${{ matrix.platform }}
           load: true
           tags: ${{ env.IMAGE_NAME }}-${{ env.LATEST_VERSION }}
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,mode=max,scope=${{ matrix.platform }}
 
       - name: Test Image
         run: docker run --rm "${IMAGE_NAME}-${LATEST_VERSION}" -e "console.log('Hello from Node.js ' + process.version)"

.github/workflows/update-current-image.yml

@@ -137,6 +137,8 @@ jobs:
           platforms: linux/${{ matrix.platform }}
           load: true
           tags: ${{ env.IMAGE_NAME }}-${{ env.NODE_VERSION }}
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,mode=max,scope=${{ matrix.platform }}
 
       - name: Test Image
         run: docker run --rm "${IMAGE_NAME}-${NODE_VERSION}" -e "console.log('Hello from Node.js ' + process.version)"
@@ -152,6 +154,7 @@ jobs:
           tags: |
             ${{ secrets.DOCKERHUB_USERNAME }}/${{ env.IMAGE_NAME }}:build-${{ github.run_id }}-${{ matrix.platform }}
             ${{ env.GHCR_IMAGE }}:build-${{ github.run_id }}-${{ matrix.platform }}
+          cache-from: type=gha,scope=${{ matrix.platform }}
 
   merge:
     needs: [check_version, build]