Attempt #1 on merging multi-platform docker builds

Author: brendan-kellam

.github/workflows/ghcr-publish.yml

@@ -9,17 +9,15 @@ on:
   # schedule:
   #   - cron: '18 10 * * *'
   push:
-    branches: [ "main" ]
+    branches: ["main"]
     # Publish semver tags as releases.
-    tags: [ 'v*.*.*' ]
+    tags: ["v*.*.*"]
   pull_request:
-    branches: [ "main" ]
+    branches: ["main"]
 
 env:
   # Use docker.io for Docker Hub if empty
-  REGISTRY: ghcr.io
-  IMAGE_NAME: ${{ github.repository }}
-
+  REGISTRY_IMAGE: ghcr.io/sourcebot
 
 jobs:
   build:
@@ -32,53 +30,73 @@ jobs:
       id-token: write
     strategy:
       matrix:
-        architecture: [amd64, arm64]
+        platform:
+          - linux/amd64
+          - linux/arm64
 
     steps:
+      - name: Prepare
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
+
       - name: Checkout repository
         uses: actions/checkout@v4
         with:
-          submodules: 'true'
-      
+          submodules: "true"
+
       # Extract metadata (tags, labels) for Docker
       # https://github.com/docker/metadata-action
       - name: Extract Docker metadata
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          images: ${{ env.REGISTRY_IMAGE }}
 
       # Install the cosign tool except on PR
       # https://github.com/sigstore/cosign-installer
       - name: Install cosign
         if: github.event_name != 'pull_request'
         uses: sigstore/[email protected]
         with:
-          cosign-release: 'v2.2.4'
+          cosign-release: "v2.2.4"
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v1
-    
+        uses: docker/setup-buildx-action@v3
+
       - name: Login to GitHub Packages Docker Registry
         uses: docker/login-action@v1
         if: github.event_name != 'pull_request'
         with:
-          registry: ${{ env.REGISTRY }}
+          registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push Docker image
-        id: build-and-push
-        uses: docker/build-push-action@v5
+        id: build
+        uses: docker/build-push-action@v6
         with:
           context: .
-          push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha
           cache-to: type=gha,mode=max
-          platforms: linux/${{ matrix.architecture }}
-      
+          platforms: ${{ matrix.platform }}
+
+      - name: Export digest
+        run: |
+          mkdir -p /tmp/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "/tmp/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v4
+        with:
+          name: digests-${{ env.PLATFORM_PAIR }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
       # Sign the resulting Docker image digest except on PRs.
       # This will only write to the public Rekor transparency log when the Docker
       # repository is public to avoid leaking data.  If you would like to publish
@@ -89,7 +107,46 @@ jobs:
         env:
           # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
           TAGS: ${{ steps.meta.outputs.tags }}
-          DIGEST: ${{ steps.build-and-push.outputs.digest }}
+          DIGEST: ${{ steps.build.outputs.digest }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+  
+  merge:
+    runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request'
+    needs:
+      - build
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: /tmp/digests
+          pattern: digests-*
+          merge-multiple: true
+      
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY_IMAGE }}
+
+      - name: Login to GitHub Packages Docker Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      
+      - name: Create manifest list and push
+        working-directory: /tmp/digests
+        run: |
+          docker buildx imagetools create $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf '${{ env.REGISTRY_IMAGE }}@sha256:%s ' *)          
+      
+      - name: Inspect image
+        run: |
+          docker buildx imagetools inspect ${{ env.REGISTRY_IMAGE }}:${{ steps.meta.outputs.version }}