ci: add l4-port-manager image to Docker build workflows

The l4-port-manager service had a Dockerfile but was missing from the
GitHub Actions build matrix, so it was never built or pushed to GHCR.

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

.github/workflows/docker-build-trusted.yml

@@ -29,6 +29,9 @@ jobs:
           - service: caddy
             dockerfile: docker/caddy/Dockerfile
             context: .
+          - service: l4-port-manager
+            dockerfile: docker/l4-port-manager/Dockerfile
+            context: .
 
     steps:
       - name: Checkout repository