github: Build and publish Docker image on release

Fixes #10760

.github/workflows/image.yml

@@ -0,0 +1,127 @@
+name: Build Docker image
+
+on:
+  push:
+
+  # Run on pull requests to verify that the image still builds correctly
+  # The image is not pushed on pull requests
+  pull_request:
+    paths:
+      - Dockerfile
+      - .github/workflows/image.yml
+
+permissions:
+  packages: write
+
+env:
+  REGISTRY_IMAGE: ghcr.io/gohugoio/hugo
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - linux/amd64
+          - linux/arm64
+    steps:
+      - name: Prepare
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
+
+      - name: Checkout
+        uses: actions/checkout@v4.1.6
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5.5.1
+        with:
+          images: ${{ env.REGISTRY_IMAGE }}
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3.0.0
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3.3.0
+
+      - name: Login to GHCR
+        # Login is only needed when the image is pushed
+        if: ${{ startsWith(github.ref, 'refs/tags') }}
+        uses: docker/login-action@v3.2.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push by digest
+        id: build
+        uses: docker/build-push-action@v5.3.0
+        with:
+          context: .
+          push: ${{ startsWith(github.ref, 'refs/tags') }}
+          platforms: ${{ matrix.platform }}
+          labels: ${{ steps.meta.outputs.labels }}
+          outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
+
+      - 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@v4.3.3
+        with:
+          name: digests-${{ env.PLATFORM_PAIR }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    runs-on: ubuntu-latest
+    needs:
+      - build
+    if: ${{ startsWith(github.ref, 'refs/tags') }}
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v4.1.7
+        with:
+          path: /tmp/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3.3.0
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5.5.1
+        with:
+          images: ${{ env.REGISTRY_IMAGE }}
+
+          flavor: |
+            latest=false
+
+          # Tag the image with the v-prefixed semver version on tags, tag with the ref on PRs
+          tags: |
+            type=semver,pattern={{raw}}
+            type=ref,event=pr
+
+      - name: Login to GHCR
+        uses: docker/login-action@v3.2.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          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 }}