Add note to 'docker' commands

Signed-off-by: Grzegorz Gwóźdź <grzegorz.gwozdz@huawei.com>

transparent-gateway-details.rst

@@ -22,8 +22,16 @@ The current manual process is quite intricated, but expect vast improvements in
 
 .. code-block:: bash
 
-   $ sudo docker pull openthread/environment:latest
-   $ sudo docker run -it --rm openthread/environment bash
+   $ docker pull openthread/environment:latest
+   $ docker run -it --rm openthread/environment bash
+
+   .. note::
+
+      You may receive an error message `permisssion denied`. You can start both 
+      of the commands with `sudo`.
+      If you don’t want to preface the `docker` command with `sudo`, create a Unix group
+      called `docker` and add users to it. When the Docker daemon starts, it creates a 
+      Unix socket accessible by members of the `docker` group. 
 
 - Inside the docker container prepare the build as follows: