[CI] Merge docker images across platforms

With the current setup, images for different platforms are pushed with the same
tag, overriding each other.  With this change, instead, images built for
different architectures are merged together.

[skip ci]

Author: giordano

.github/workflows/Container.yml

@@ -1,3 +1,8 @@
+# Based on
+# * <https://github.com/NumericalEarth/breeze-docker-images/blob/f06a3da11b2c7edaaf0a8e32c133270eb4103b1c/.github/workflows/DockerPublish.yml>
+# * <https://docs.github.com/en/actions/publishing-packages/publishing-docker-images>
+# * <https://docs.docker.com/build/ci/github-actions/multi-platform/#distribute-build-across-multiple-runners>.
+
 name: Publish Docker container
 
 on:
@@ -18,12 +23,20 @@ on:
     branches:
       - master
 
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
 jobs:
-  push_to_registry:
-    name: Container for ${{ matrix.platform }} - Julia ${{ matrix.julia }} - CUDA ${{ matrix.cuda }}
+  build:
+    name: Build Container for ${{ matrix.platform }} - Julia ${{ matrix.julia }} - CUDA ${{ matrix.cuda }}
     permissions:
       contents: read
       packages: write
+      attestations: write
+      # This is used to complete the identity challenge
+      # with sigstore/fulcio when running outside of PRs.
+      id-token: write
 
     strategy:
       matrix:
@@ -46,6 +59,12 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
+      # Docker is terrible and doesn't like uppercase image names.
+      - name: Lowercase image name
+        run: |
+          IMAGE_NAME=$(echo ${IMAGE_NAME} | tr A-Z a-z)
+          echo "IMAGE_NAME=${IMAGE_NAME}" >> "${GITHUB_ENV}"
+
       - name: Check out the repo
         uses: actions/checkout@v6
 
@@ -90,37 +109,105 @@ jobs:
       - name: Log in to registry
         uses: docker/login-action@v3
         with:
-          registry: ghcr.io
+          registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Extract metadata
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ghcr.io/${{ github.repository }}
-          tags: |
-              type=raw,value=${{ steps.pkg.outputs.name }}-julia${{ matrix.julia }}-cuda${{ steps.cuda.outputs.major }}
-              type=raw,value=${{ steps.pkg.outputs.name }},enable=${{ matrix.default == true && (github.ref_type == 'tag' || inputs.tag != '') }}
-              type=raw,value=latest,enable=${{ matrix.default == true && (github.ref_type == 'tag' || (inputs.tag != '' && inputs.mark_as_latest)) }}
-              type=raw,value=dev,enable=${{ matrix.default == true && github.ref_type == 'branch' && inputs.tag == '' }}
-          labels: |
-              org.opencontainers.image.version=${{ steps.pkg.outputs.version }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Build and push image
+      - name: Build and push
+        id: build
         uses: docker/build-push-action@v6
         with:
           context: .
           push: true
           provenance: false # the build fetches the repo again, so provenance tracking is not useful
           platforms: ${{ matrix.platform }}
-          tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}
+          tags: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.pkg.outputs.name }}-julia${{ matrix.julia }}-cuda${{ steps.cuda.outputs.major }}
+          labels: |
+              org.opencontainers.image.version=${{ steps.pkg.outputs.version }}
           build-args: |
             JULIA_VERSION=${{ matrix.julia }}
             CUDA_VERSION=${{ matrix.cuda }}
             PACKAGE_SPEC=CUDA#${{ steps.pkg.outputs.ref }}
             JULIA_CPU_TARGET=${{ steps.cpu_target.outputs.target }}
+
+      - name: Export digest
+        id: export-digest
+        run: |
+          mkdir -p /tmp/digests
+          digest_name=${{ steps.pkg.outputs.name }}-julia${{ matrix.julia }}-cuda${{ steps.cuda.outputs.major }}-$(echo ${{ matrix.platform }} | tr / -)
+          echo "digest_name=${digest_name}" | tee "${GITHUB_OUTPUT}"
+          echo "${{ steps.build.outputs.digest }}" > "/tmp/digests/${digest_name}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v6
+        with:
+          name: digests-${{ steps.export-digest.outputs.digest_name }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      # This is used to complete the identity challenge
+      # with sigstore/fulcio when running outside of PRs.
+      id-token: write
+    if: github.event_name != 'pull_request'
+
+    steps:
+      # Docker is terrible and doesn't like uppercase image names.
+      - name: Lowercase image name
+        run: |
+          IMAGE_NAME=$(echo ${IMAGE_NAME} | tr A-Z a-z)
+          echo "IMAGE_NAME=${IMAGE_NAME}" >> "${GITHUB_ENV}"
+
+      - name: Download digests
+        uses: actions/download-artifact@v7
+        with:
+          path: /tmp/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      # https://github.com/docker/setup-buildx-action
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      # Log into the registry.
+      # https://github.com/docker/login-action
+      - name: Log into registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Create manifest lists for each tag
+        run: |
+          for file in /tmp/digests/*; do
+            TAG=${file##*/}                        # e.g. "dev-julia1.11-cuda11-linux-amd64"
+            VARIANT=$(echo ${TAG} | cut -d- -f1-3) # e.g. "dev-julia1.11-cuda11"
+            # buildx imagetools needs: -t registry/image:tag and sources
+            SOURCES=""
+            for f in /tmp/digests/${VARIANT}-*; do
+              # each f contains a digest per arch
+              DIG=$(cat "${f}")
+              PLATFORM=${f##*/}            # e.g. "docs-linux-amd64"
+              SOURCES="${SOURCES} ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${DIG}"
+            done
+            docker buildx imagetools create \
+              --tag ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${VARIANT} \
+              ${SOURCES}
+          done