Clear uname/gname when creating tar archive

To ensure that the cache from docker command client
can be reused, we must clear group/user name as the docker
client does.

Together with golang/go#20150 this fixes docker#998

docker/utils/utils.py

@@ -98,6 +98,8 @@ def create_archive(root, files=None, fileobj=None, gzip=False):
         files = build_file_list(root)
     for path in files:
         i = t.gettarinfo(os.path.join(root, path), arcname=path)
+        i.uname = ''
+        i.gname = ''
         if i is None:
             # This happens when we encounter a socket file. We can safely
             # ignore it and proceed.