Docker Dashboard Doc: Username and Password (#3046)

Co-authored-by: H. Árkosi Róbert <robreg@zsurob.hu>

guides/getting_started_command_line.rst

@@ -39,6 +39,9 @@ If you want to use `docker-compose` instead, here's a sample file:
         restart: always
         privileged: true
         network_mode: host
+        environment:
+          - USERNAME=test 
+          - PASSWORD=ChangeMe
 
 .. note::