Containers: wait for up/down streams to finish resolving the connectI…

…mpl promise

@kenton suggested adding this line to fix the issues we were finding
with the containers integration.

src/workerd/api/container.c++

@@ -199,6 +199,7 @@ class Container::TcpPortWorkerInterface final: public WorkerInterface {
     });
 
     co_await pipeline.ignoreResult();
+    co_await kj::joinPromisesFailFast(kj::arr(kj::mv(upPumpTask), kj::mv(downPumpTask)));
   }
 };