transparent-gateway-details.rst: Fix Radio-Co Processor firmware docker build

The instructions for docker build do not cover non-root error

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

transparent-gateway-details.rst

@@ -26,6 +26,13 @@ The current manual process is quite intricated, but expect vast improvements in
    $ docker pull openthread/environment:latest
    $ docker run -it --rm openthread/environment bash
 
+.. note::
+
+      You may receive `permission denied` error message.
+      Solution is available on the
+      `docker docs website <https://docs.docker.com/engine/install/linux-postinstall/#manage-docker-as-a-non-root-user>`_ 
+      
+
 - Inside the docker container prepare the build as follows:
 
 .. code-block:: bash