Add docker image build job

.github/workflows/docker.yml

@@ -0,0 +1,118 @@
+name: Docker
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*.*.*"]
+  pull_request:
+    branches: [main]
+  # Manual republish (e.g. base-image refresh without a version bump).
+  # Must be dispatched FROM A TAG REF: metadata-action only derives semver
+  # image tags from the git tag, so a branch dispatch is rejected below.
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  # Fail fast with a clear message instead of silently pushing a bare
+  # `latest` (or nothing) when someone dispatches from a branch.
+  guard-dispatch:
+    if: github.event_name == 'workflow_dispatch' && !startsWith(github.ref, 'refs/tags/v')
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo "::error::Manual runs must be dispatched from a v*.*.* tag ref (use the 'Run workflow' ref selector). Branch refs produce no semver image tags."
+          exit 1
+
+  # Catch tag/code drift before anything is published (root.go has drifted
+  # from tags before).
+  verify-version:
+    if: startsWith(github.ref, 'refs/tags/v')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Check tag matches cmd/root.go version
+        run: |
+          TAG_VERSION="${GITHUB_REF_NAME#v}"
+          CODE_VERSION=$(sed -nE 's/^[[:space:]]*version[[:space:]]*=[[:space:]]*"([0-9]+\.[0-9]+\.[0-9]+)".*/\1/p' cmd/root.go)
+          echo "tag=v${TAG_VERSION} code=${CODE_VERSION}"
+          if [ "$TAG_VERSION" != "$CODE_VERSION" ]; then
+            echo "::error::Git tag v${TAG_VERSION} does not match version \"${CODE_VERSION}\" in cmd/root.go. Bump cmd/root.go before tagging."
+            exit 1
+          fi
+
+  # PR / main validation: build only, single platform (multi-arch under QEMU
+  # roughly doubles CI time for no extra signal; tag builds cover arm64).
+  build:
+    if: github.event_name == 'pull_request' || (github.event_name == 'push' && github.ref == 'refs/heads/main')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build (no push)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: false
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+  # Publish on version tags (and manual dispatch from a tag ref).
+  # No fork-secrets concern: this never runs for pull_request events.
+  publish:
+    if: startsWith(github.ref, 'refs/tags/v')
+    needs: verify-version
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      # `latest` is applied automatically on semver tags (latest=auto).
+      # Caveat: it follows the most recently pushed tag, not the highest
+      # version — we only tag forward, never backport.
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: karust/openserp
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
+      # The pinned chromedp/headless-shell digest is a multi-arch OCI index
+      # (amd64 + arm64/v8), and the builder stage cross-compiles via
+      # $BUILDPLATFORM, so only the small final-stage apt layer runs under QEMU.
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max