x11docker sudo-less xorg authentication

x11docker_sudoless_xorg_auth.md

Your configuration seems not to allow to start
  a second core Xorg server from within X. Option --xorg will probably fail.
  (Per default, only root or console users are allowed to run an Xorg server).

  Possible solutions:
  1.) Install one of nested X servers 'Xephyr', 'Xnest' or 'nxagent'.
      For --gpu support: install 'weston' and 'Xwayland'.
  2.) Switch to console tty1...tty6 with <CTRL><ALT><F1>...<F6>
      and start x11docker there.
  3.) Run x11docker as root.
  4.) Edit file '/etc/X11/Xwrapper.config' and replace line:
          allowed_users=console
      with lines
          allowed_users=anybody
          needs_root_rights=yes
      If the file does not exist already, you can create it.
      On Ubuntu 16.04 and debian 9 you need package xserver-xorg-legacy.