ci: publish operator image to GHCR on push/tag

New workflow builds the multi-stage Dockerfile and pushes to
ghcr.io/michelecampi/vllm-coldstart-operator. Tags: short SHA, branch,
semver from v* tags, and latest on main. Builds (no push) on PRs to catch
Dockerfile regressions. Separate from ci.yml to keep the test pipeline
independent.

Author: MicheleCampi

.github/workflows/release-image.yml

@@ -0,0 +1,59 @@
+name: Release image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+    branches: [main]
+
+# Cancel superseded runs on the same ref.
+concurrency:
+  group: release-image-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  image:
+    name: build + push (ghcr)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      # Login only when we intend to push (not on PRs).
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          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=sha,prefix=
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max