Improving documentation for --url parameter

Adding documentation regarding the default behaviour of "cli_run" in adapter "docker.py", which is called when --url should fetch from docker-hub.

datalad_container/containers_add.py

@@ -138,7 +138,8 @@ class ContainersAdd(Interface):
             the rest of the URL will be interpreted as the argument to
             'docker pull', the image will be saved to a location
             specified by `name`, and the call format will be auto-configured
-            to run docker, unless overwritten.""",
+            to run docker, unless overwritten. The auto-configured call to docker 
+            run mounts the cwd to '/tmp' and sets the working directory to '/tmp'.""",
             metavar="URL",
             constraints=EnsureStr() | EnsureNone(),
         ),