Document root homedir (#2079)

* Document root homedir

* Add link to issue

Author: mathbunnyru

docs/using/common.md

@@ -52,6 +52,11 @@ You do so by passing arguments to the `docker run` command.
       quay.io/jupyter/base-notebook
   ```
 
+  ```{note}
+  If you set `NB_USER` to `root`, the `root` home dir will be set to `/home/root`.
+  See discussion [here](https://github.com/jupyter/docker-stacks/issues/2042).
+  ```
+
 - `-e NB_UID=<numeric uid>` - Instructs the startup script to switch the numeric user ID of `${NB_USER}` to the given value.
   The default value is `1000`.
   This feature is useful when mounting host volumes with specific owner permissions.