Fix publish workflow: use 'file' instead of 'dockerfile' parameter

docker/build-push-action@v6 renamed the parameter from 'dockerfile' to
'file'. The backend succeeded by luck (default Dockerfile matched), but
nginx failed because the Dockerfile isn't in the root context directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/publish.yml

@@ -49,7 +49,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: ./backend
-          dockerfile: backend/Dockerfile
+          file: backend/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
@@ -92,7 +92,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
-          dockerfile: nginx/Dockerfile
+          file: nginx/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}