Add GHCR publish job to CI workflow

Publishes Docker image to ghcr.io on every push to main,
after all existing checks (lint, import-check, security, docker) pass.
Tags with latest and sha-<commit>.

Author: bk86a

.github/workflows/ci.yml

@@ -8,6 +8,7 @@ on:
 
 permissions:
   contents: read
+  packages: write
 
 jobs:
   lint:
@@ -54,3 +55,28 @@ jobs:
           user=$(docker run --rm postalcode2nuts whoami)
           echo "Container user: $user"
           [ "$user" = "appuser" ] || exit 1
+
+  publish:
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    needs: [lint, import-check, security, docker]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - uses: docker/metadata-action@v5
+        id: meta
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=raw,value=latest
+            type=sha
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}