Disable SSR and hold the process open

Gradio 6 defaults to SSR, which fronts the public port with a Node proxy and
serves Python on 7861 behind it — routes grafted onto the Python app were
unreachable from outside. block_thread() also returned straight away and the
container exited, so wait explicitly.

app.py

@@ -269,9 +269,21 @@ if __name__ == "__main__":
         server_port=int(os.environ.get("PORT", 7860)),
         prevent_thread_lock=True,
         show_error=True,
+        # SSR is Gradio 6's default and puts a Node proxy on the public port,
+        # serving Python on 7861 behind it. Routes grafted onto the Python app
+        # are then unreachable from outside, so the REST API has to have the
+        # public port to itself.
+        ssr_mode=False,
     )
     demo.app.router.routes[0:0] = app.router.routes
     print("REST routes attached: " +
           ", ".join(sorted(r.path for r in app.router.routes if hasattr(r, "methods"))),
           flush=True)
-    demo.block_thread()
+    # block_thread() returned immediately here and the container exited; an
+    # explicit wait keeps the process alive for as long as the server runs.
+    try:
+        demo.block_thread()
+    except KeyboardInterrupt:
+        pass
+    else:
+        threading.Event().wait()