feat: add GHCR container image publishing workflow

Add a GitHub Actions workflow that builds and pushes multi-arch
(amd64/arm64) container images to GHCR on pushes to main and version
tags. Includes build caching and SLSA provenance attestation.

Update docker-compose.yml to pull from GHCR by default.

Closes #6

Author: raffaelschneider

.github/workflows/publish.yml

@@ -0,0 +1,66 @@
+name: Publish Container Image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    name: Build & Push to GHCR
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      id-token: write
+
+    steps:
+      - 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 GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha
+
+      - name: Build and push
+        id: 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
+
+      - name: Generate artifact attestation
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.push.outputs.digest }}
+          push-to-registry: true

docker-compose.yml

@@ -1,6 +1,8 @@
 services:
   app:
-    build: .
+    image: ghcr.io/zentinelproxy/zentinel-control-plane:main
+    # To build locally instead, comment out `image` and uncomment:
+    # build: .
     ports:
       - "4000:4000"
     environment: