Distributed multiarch builds

.github/workflows/docker.yml

@@ -4,61 +4,66 @@ on:
   workflow_call:
     inputs:
       publish:
-        description: 'Whether to sign and publish the image using cosign and GHCR'
+        description: "Whether to sign and publish the image using cosign and GHCR"
         required: true
         type: boolean
       dockerfile:
-        description: 'Path to the Dockerfile to build'
+        description: "Path to the Dockerfile to build"
         required: false
         type: string
       context:
-        description: 'Path to the build context'
+        description: "Path to the build context"
         required: false
         type: string
       image_name:
-        description: 'Name of the image to build'
+        description: "Name of the image to build"
         required: false
         type: string
+      artifact_name:
+        description: "Name prefix for digest artifacts (defaults to 'digests'). Useful when running multiple docker builds in the same workflow."
+        required: false
+        type: string
+        default: "digests"
 
 env:
   # Use docker.io for Docker Hub if empty
   REGISTRY: ghcr.io
   # github.repository as <account>/<repo>
   IMAGE_NAME: ${{ github.repository }}
+  IMAGE_NAME_LOWER: ""
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
+  # Build docker image for each architecture in parallel
+  build-arch:
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - linux/amd64
+          - linux/arm64
+
+    runs-on: ${{ matrix.platform == 'linux/arm64' && 'ubuntu-24.04-arm' || 'ubuntu-24.04' }}
     permissions:
       contents: read
       packages: write
-      # This is used to complete the identity challenge
-      # with sigstore/fulcio when running outside of PRs.
       id-token: write
 
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      # Install the cosign tool except on PR
-      # https://github.com/sigstore/cosign-installer
-      - name: Install cosign
-        if: inputs.publish
-        uses: sigstore/cosign-installer@v3
-          
+      - name: Prepare
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
+          echo "IMAGE_NAME_LOWER=$(echo '${{ inputs.image_name || env.IMAGE_NAME }}' | tr '[:upper:]' '[:lower:]')" >> $GITHUB_ENV
+
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
-        with:
-          platforms: arm64
 
-      # Workaround: https://github.com/docker/build-push-action/issues/461
-      - name: Setup Docker buildx
+      - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
-        with:
-          platforms: linux/amd64,linux/arm64
 
-      # Login against a Docker registry except on PR
-      # https://github.com/docker/login-action
       - name: Log into registry ${{ env.REGISTRY }}
         if: inputs.publish
         uses: docker/login-action@v3
@@ -67,41 +72,115 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      # Extract metadata (tags, labels) for Docker
-      # https://github.com/docker/metadata-action
       - name: Extract Docker metadata
         id: meta
         uses: docker/metadata-action@v5
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
-          images: ${{ env.REGISTRY }}/${{ inputs.image_name || env.IMAGE_NAME }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }}
 
-      # Build and push Docker image with Buildx (don't push on PR)
-      # https://github.com/docker/build-push-action
-      - name: Build and push Docker image
-        id: build-and-push
-        uses: docker/build-push-action@v5
+      - name: Build and push by digest
+        id: build
+        uses: docker/build-push-action@v6
         with:
           context: ${{ inputs.context || '.' }}
-          push: ${{ inputs.publish }}
-          tags: ${{ steps.meta.outputs.tags }}
+          file: ${{ inputs.dockerfile || 'Dockerfile' }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
+          outputs: type=image,name=${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }},push-by-digest=true,name-canonical=true,push=${{ inputs.publish }}
           cache-from: type=gha
           cache-to: type=gha,mode=max
-          platforms: linux/amd64,linux/arm64
-          file: ${{ inputs.dockerfile || 'Dockerfile' }}
 
-      # Sign the resulting Docker image digest except on PRs.
+      - name: Export digest
+        if: inputs.publish
+        run: |
+          mkdir -p ${{ runner.temp }}/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "${{ runner.temp }}/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        if: inputs.publish
+        uses: actions/upload-artifact@v4
+        with:
+          name: ${{ inputs.artifact_name }}-${{ env.PLATFORM_PAIR }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  # Merge all architecture images into a single manifest
+  merge:
+    runs-on: ubuntu-latest
+    if: inputs.publish
+    needs:
+      - build-arch
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+
+    steps:
+      - name: Prepare
+        run: |
+          echo "IMAGE_NAME_LOWER=$(echo '${{ inputs.image_name || env.IMAGE_NAME }}' | tr '[:upper:]' '[:lower:]')" >> $GITHUB_ENV
+
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: ${{ inputs.artifact_name }}-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log into registry ${{ env.REGISTRY }}
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }}
+
+      - name: Create manifest list and push
+        id: manifest
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          docker buildx imagetools create $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+              $(printf '${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }}@sha256:%s ' *)
+
+      - name: Inspect image
+        id: inspect
+        run: |
+          docker buildx imagetools inspect ${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }}:${{ steps.meta.outputs.version }} --raw | jq -r '.manifests[0].digest'
+          digest=$(docker buildx imagetools inspect ${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LOWER }}:${{ steps.meta.outputs.version }} --raw | sha256sum | awk '{print "sha256:" $1}')
+          echo "digest=${digest}" >> $GITHUB_OUTPUT
+
+      # Install the cosign tool
+      # https://github.com/sigstore/cosign-installer
+      - name: Install cosign
+        uses: sigstore/cosign-installer@v3
+
+      # Sign the resulting Docker image digest.
       # 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
       # transparency data even for private images, pass --force to cosign below.
       # https://github.com/sigstore/cosign
       - name: Sign the published Docker image
-        if: inputs.publish
         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: ${{ steps.inspect.outputs.digest }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+        run: |
+          images=""
+          for tag in ${TAGS}; do
+            images+="${tag}@${DIGEST} "
+          done
+          echo "${images}" | xargs -n 1 cosign sign --yes