Unnecessary warning printed during 'register' in system-mode

root@docker1:~# gitlab-runner register
Running in system-mode.                            
WARNING: The user-mode requires you to manually start builds processing: 
WARNING: $ gitlab-runner run                       
                                                   
Please enter the gitlab-ci coordinator URL (e.g. https://gitlab.com/ci):
...

This warning shouldn't be printed when running as root.