diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index 18e2c0868a..135743a2bf 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -301,7 +301,7 @@ An alternative method to set up permissions is by adding the current user to
 .. code::
 
   $ sudo groupadd docker
-  $ sudo usermod $USER -G docker
+  $ sudo usermod $USER -a -G docker
   $ sudo chown :docker /var/run/docker.sock
 
 Note that any one of above configurations makes it possible for the user to