docker: fix nw_service not available error

There is race condition between container start and ssh-server service ready in container,
which may possible make `nw_service` not available.

Make `nw_service` for docker not static allows `target` to wait for the available
of `nw_service`, rather than raise an exception.

Signed-off-by: Larry Shen <[email protected]>

Author: atline

labgrid/resource/docker.py

@@ -108,6 +108,7 @@ def on_client_bound(self, client):
             nw_service = target_factory.make_resource(
                 self.target, "NetworkService",
                 service_name, network_service)
+            nw_service.parent = self
             if client.container_name not in self._nw_services:
                 self._nw_services[client.container_name] = list()
             self._nw_services[client.container_name].append(nw_service)