fix(docker): default Gateway to a single worker to prevent multi-worker breakage (#3475)

The default `make up` started the Gateway with `--workers 4`, but run state
(RunManager and the stream bridge) is held in-process and nginx uses no sticky
sessions. With the default config, same-run requests scatter across workers that
each keep their own run state, breaking run cancellation (409), SSE reconnect
(hangs on heartbeats), multitask de-duplication, and IM channels (duplicate
replies). The shared cross-worker stream bridge does not exist yet.

Default GATEWAY_WORKERS to 1 so the out-of-the-box deployment is correct,
document the single-worker boundary in the README, and add a regression test
pinning the default while keeping it overridable. This is a stop-gap, not a
multi-worker implementation; the full fix (shared run state + stream bridge) is
tracked in #3191.

Refs #3239, #3260

docker/docker-compose.yaml

@@ -72,7 +72,13 @@ services:
         UV_INDEX_URL: ${UV_INDEX_URL:-https://pypi.org/simple}
         UV_EXTRAS: ${UV_EXTRAS:-}
     container_name: deer-flow-gateway
-    command: sh -c "cd backend && PYTHONPATH=. uv run uvicorn app.gateway.app:app --host 0.0.0.0 --port 8001 --workers ${GATEWAY_WORKERS:-4}"
+    # Gateway hosts the agent runtime with in-process RunManager + StreamBridge
+    # singletons -- run state lives in this worker's memory. Default to a single
+    # worker: with >1 worker and no nginx sticky sessions, run cancel, SSE
+    # reconnect, request dedup, and per-worker IM channel services all break
+    # across workers until a shared (e.g. redis) stream bridge lands, which is
+    # not yet implemented. Override GATEWAY_WORKERS only once that is in place.
+    command: sh -c "cd backend && PYTHONPATH=. uv run uvicorn app.gateway.app:app --host 0.0.0.0 --port 8001 --workers ${GATEWAY_WORKERS:-1}"
     volumes:
       - ${DEER_FLOW_CONFIG_PATH}:/app/backend/config.yaml:ro
       - ${DEER_FLOW_EXTENSIONS_CONFIG_PATH}:/app/backend/extensions_config.json:ro