Fix connection to websocket with last docker release

Fix https://github.com/GNS3/gns3-gui/issues/1994

Signed-off-by: Julien Duponchelle <julien@gns3.net>

gns3server/compute/docker/__init__.py

@@ -161,7 +161,7 @@ class Docker(BaseManager):
         :returns: Websocket
         """
 
-        url = "http://docker/" + path
+        url = "http://docker/v" + DOCKER_MINIMUM_API_VERSION + "/" + path
         connection = yield from aiohttp.ws_connect(url,
                                                    connector=self.connector(),
                                                    origin="http://docker",