chore(workflows): update Docker publish workflow for multi-platform support

- Changed the runner from `anduro-runner` to `ubuntu-latest` for parralel jobs compatibility.
- Added a strategy for multi-platform builds targeting `linux/amd64` and `linux/arm64`.
- Set up QEMU for cross-platform builds and adjusted the build step to push images by digest.
- Introduced a new merge job to create and push a multi-arch manifest after building images.
- Removed the conditional installation of the cosign tool and streamlined the signing process for published images.

Author: michaelgpt

.github/workflows/docker-publish.yml

@@ -26,7 +26,11 @@ env:
 
 jobs:
   build:
-    runs-on: anduro-runner
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform: [linux/amd64, linux/arm64]
     permissions:
       contents: read
       packages: write
@@ -38,18 +42,15 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      # Install the cosign tool except on PR
-      # https://github.com/sigstore/cosign-installer
-      - name: Install cosign
-        if: github.event_name != 'pull_request'
-        uses: sigstore/cosign-installer@v3.5.0
-
       # Set up BuildKit Docker container builder to be able to build
       # multi-platform images and export cache
       # https://github.com/docker/setup-buildx-action
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@f95db51fddba0c2d1ec667646a06c2ce06100226 # v3.0.0
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       # Login against a Docker registry except on PR
       # https://github.com/docker/login-action
       - name: Log into registry ${{ env.REGISTRY }}
@@ -68,32 +69,84 @@ jobs:
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
 
-      # Build and push Docker image with Buildx (don't push on PR)
+      # Build and push per-arch image by digest (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@v6 # v5.0.0
+      - name: Build and push by digest (${{ matrix.platform }})
+        id: build
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./etc/Dockerfile
           push: ${{ github.event_name != 'pull_request' }}
-          tags: ${{ steps.meta.outputs.tags }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
-          platforms: linux/amd64,linux/arm64
           cache-from: type=gha
           cache-to: type=gha,mode=max
+          outputs: type=image,name=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }},push-by-digest=true
+
+      - name: Export digest
+        if: ${{ github.event_name != 'pull_request' }}
+        run: |
+          mkdir -p /tmp/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "/tmp/digests/${digest#sha256:}"
 
-      # 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
-      # transparency data even for private images, pass --force to cosign below.
-      # https://github.com/sigstore/cosign
-      - name: Sign the published Docker image
+      - name: Upload digest
         if: ${{ github.event_name != 'pull_request' }}
+        uses: actions/upload-artifact@v4
+        with:
+          name: digests-${{ matrix.platform }}
+          path: /tmp/digests/*
+
+  merge:
+    if: ${{ github.event_name != 'pull_request' }}
+    needs: build
+    runs-on: anduro-runner
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+    steps:
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@f95db51fddba0c2d1ec667646a06c2ce06100226 # v3.0.0
+
+      - name: Log into registry ${{ env.REGISTRY }}
+        uses: docker/login-action@343f7c4344506bcbf9b4de18042ae17996df046d # v3.0.0
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          pattern: digests-*
+          path: /tmp/digests
+          merge-multiple: true
+
+      # Extract metadata (tags, labels) for Docker
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Create and push multi-arch manifest
+        run: |
+          tags="${{ steps.meta.outputs.tags }}"
+          for tag in $tags; do
+            args=""
+            for file in /tmp/digests/*; do
+              args="$args ${tag}@sha256:$(basename "$file")"
+            done
+            docker buildx imagetools create -t "$tag" $args
+          done
+
+      # Install the cosign tool
+      - name: Install cosign
+        uses: sigstore/cosign-installer@v3.5.0
+
+      - name: Sign the published Docker images
         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: ${{ steps.build-and-push.outputs.digest }}
-        # This step uses the identity token to provision an ephemeral certificate
-        # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}