fix: use docker label for build job (runner has docker-cli)

.gitea/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
   # Docker build job (depends on backend and frontend builds)
   docker-build:
     needs: [backend-build, frontend-build]
-    runs-on: ubuntu-latest
+    runs-on: docker
     if: github.ref == 'refs/heads/main'
     steps:
       - name: Checkout code