Update DOCKER_REGISTRY_URL to the recommended one
@@ -11,7 +11,7 @@ from tornado.httputil import url_concat
from traitlets.config import LoggingConfigurable
from traitlets import Dict, Unicode, default
-DEFAULT_DOCKER_REGISTRY_URL = "https://registry.hub.docker.com"
+DEFAULT_DOCKER_REGISTRY_URL = "https://registry-1.docker.io"
DEFAULT_DOCKER_AUTH_URL = "https://index.docker.io/v1"
