ci: publish a multi-arch container image to GHCR

.github/workflows/image.yml

@@ -0,0 +1,119 @@
+# Publish the walgit container image (Containerfile) to GitHub Container Registry as
+# ghcr.io/<owner>/<repo>, for linux/amd64 and linux/arm64. Each architecture builds
+# natively on a runner of its own architecture — no emulation — and the two are merged
+# into one multi-arch tag.
+#
+# Publishes the repository's default branch (tags: latest, <branch>, sha-<short>) and
+# version tags v* (tags: <version>, <major>.<minor>). Pull requests build both
+# architectures without pushing, so a broken Containerfile shows up before merge.
+name: image
+
+on:
+  push:
+    branches: ["**"]
+    tags: ["v*"]
+  pull_request:
+    paths: [Containerfile, Cargo.toml, Cargo.lock, rust-toolchain.toml, "crates/**", "web/**", .github/workflows/image.yml]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: image-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+env:
+  IMAGE: ghcr.io/${{ github.repository }}
+  PUBLISH: ${{ github.event_name != 'pull_request' && (github.ref == format('refs/heads/{0}', github.event.repository.default_branch) || startsWith(github.ref, 'refs/tags/v')) }}
+
+jobs:
+  build:
+    # Branch pushes other than the default branch have nothing to publish.
+    if: github.event_name != 'push' || github.ref == format('refs/heads/{0}', github.event.repository.default_branch) || startsWith(github.ref, 'refs/tags/v')
+    strategy:
+      fail-fast: true
+      matrix:
+        include:
+          - { platform: linux/amd64, runner: ubuntu-24.04 }
+          - { platform: linux/arm64, runner: ubuntu-24.04-arm }
+    runs-on: ${{ matrix.runner }}
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v5
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        if: env.PUBLISH == 'true'
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.IMAGE }}
+      - id: build
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Containerfile
+          platforms: ${{ matrix.platform }}
+          build-args: WALGIT_BUILD_SHA=${{ github.sha }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,mode=max,scope=${{ matrix.platform }}
+          outputs: ${{ env.PUBLISH == 'true' && format('type=image,name={0},push-by-digest=true,name-canonical=true,push=true', env.IMAGE) || 'type=cacheonly' }}
+      - name: Record the digest
+        if: env.PUBLISH == 'true'
+        run: |
+          mkdir -p "${{ runner.temp }}/digests"
+          d="${{ steps.build.outputs.digest }}"
+          touch "${{ runner.temp }}/digests/${d#sha256:}"
+      - uses: actions/upload-artifact@v4
+        if: env.PUBLISH == 'true'
+        with:
+          name: digest-${{ strategy.job-index }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  publish:
+    needs: build
+    if: github.event_name != 'pull_request' && (github.ref == format('refs/heads/{0}', github.event.repository.default_branch) || startsWith(github.ref, 'refs/tags/v'))
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/download-artifact@v5
+        with:
+          pattern: digest-*
+          merge-multiple: true
+          path: ${{ runner.temp }}/digests
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.IMAGE }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=ref,event=branch
+            type=sha,prefix=sha-
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+      - name: Merge the architectures into one tag
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          mapfile -t tags < <(jq -r '.tags[] | "-t", .' <<< "$DOCKER_METADATA_OUTPUT_JSON")
+          sources=()
+          for d in *; do sources+=("${IMAGE}@sha256:${d}"); done
+          docker buildx imagetools create "${tags[@]}" "${sources[@]}"
+      - name: Inspect
+        run: docker buildx imagetools inspect "${{ env.IMAGE }}:sha-${GITHUB_SHA::7}"