Fix black screen with Qt app in Docker container

Fix #556

gns3server/modules/docker/docker_vm.py

@@ -302,6 +302,7 @@ class DockerVM(BaseVM):
 
         if self._console_type == "vnc":
             yield from self._start_vnc()
+            params["Env"].append("QT_GRAPHICSSYSTEM=native")  # To fix a Qt issue: https://github.com/GNS3/gns3-server/issues/556
             params["Env"].append("DISPLAY=:{}".format(self._display))
             params["HostConfig"]["Binds"].append("/tmp/.X11-unix/:/tmp/.X11-unix/")