address review: wait for webpack in nginx, probe manifest.json

Move the wait-for-dev-server logic out of a superset-node healthcheck
and into an nginx startup wait loop, and probe manifest.json (served
directly by webpack) instead of / (which proxies to the backend).

- Healthcheck on / was passing on backend-driven redirects and failing
  when the backend wasn't up yet; manifest.json is served by webpack
  itself at its devMiddleware publicPath and only exists after the
  first compile, which is exactly the signal we want.
- Waiting in nginx (via curl to host.docker.internal:9000) works for
  both BUILD_SUPERSET_FRONTEND_IN_DOCKER=true (dev server inside the
  superset-node container) and =false (dev server run on the host),
  so the "skip" workflow is preserved — no breaking change.
- Drops the superset-node healthcheck and the service_healthy gate
  on nginx, since nginx now polls the upstream directly.

docker-compose.yml

@@ -61,6 +61,22 @@ services:
     volumes:
       - ./docker/nginx/nginx.conf:/etc/nginx/nginx.conf:ro
       - ./docker/nginx/templates:/etc/nginx/templates:ro
+    # Wait for the webpack dev server's manifest.json to be served before
+    # starting nginx. This prevents 404s on static assets at startup. The
+    # probe targets host.docker.internal so it works regardless of whether
+    # the dev server runs in the superset-node container
+    # (BUILD_SUPERSET_FRONTEND_IN_DOCKER=true, the default) or directly on
+    # the host (BUILD_SUPERSET_FRONTEND_IN_DOCKER=false).
+    command:
+      - /bin/bash
+      - -c
+      - |
+        echo "Waiting for webpack dev server at host.docker.internal:9000/static/assets/manifest.json..."
+        until curl -sf -o /dev/null http://host.docker.internal:9000/static/assets/manifest.json; do
+          sleep 2
+        done
+        echo "Webpack dev server is ready; starting nginx."
+        exec nginx -g 'daemon off;'
 
   redis:
     image: redis:7