feat(ci): publish Docker image to GitHub Container Registry

Porter Robot Docker image now published to ghcr.io/virtusco/porter-robot
on every push to main/prototype. Shows up under VirtusCo Packages tab.

- Tags: version, latest (main only), commit SHA
- Still saves .tar.gz artifact for GitHub Releases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: austin207

.github/workflows/build-release.yml

@@ -28,20 +28,50 @@ jobs:
           echo "version=$VER" >> "$GITHUB_OUTPUT"
 
   build-docker:
-    name: Build Docker Image
+    name: Build & Publish Docker Image
     runs-on: ubuntu-latest
     needs: version
+    permissions:
+      contents: read
+      packages: write
+    env:
+      REGISTRY: ghcr.io
+      IMAGE_NAME: virtusco/porter-robot
     steps:
       - uses: actions/checkout@v4
         with:
           lfs: true
-      - name: Build production image
-        run: |
-          docker build -f docker/Dockerfile.prod \
-            -t porter-robot:${{ needs.version.outputs.version }} .
-      - name: Save image
+
+      - name: Log in to GitHub Container Registry
+        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=raw,value=${{ needs.version.outputs.version }}
+            type=raw,value=latest,enable=${{ github.ref_name == 'main' }}
+            type=sha,prefix=
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: docker/Dockerfile.prod
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+
+      - name: Save image for release artifacts
         run: |
-          docker save porter-robot:${{ needs.version.outputs.version }} | gzip > porter-robot-${{ needs.version.outputs.version }}.tar.gz
+          docker pull ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ needs.version.outputs.version }}
+          docker save ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ needs.version.outputs.version }} | gzip > porter-robot-${{ needs.version.outputs.version }}.tar.gz
       - uses: actions/upload-artifact@v4
         with:
           name: docker-image