gentoo-openrc: Break out OpenRC configuration into a separate Dockerfile

gentoo-openrc: Break out OpenRC configuration into a separate Dockerfile

Gentoo users could prefer another process manager, and we want them to
have a clean location for them to swap in their systemd (or whatever)
Dockerfile.  The net stuff is new, and with it we no longer get issues
like:

   * Bringing up network interface lo ...
  SIOCSIFADDR: Operation not permitted
  SIOCSIFFLAGS: Operation not permitted
  SIOCSIFNETMASK: Operation not permitted
  SIOCADDRT: Operation not permitted                  [ !! ]
   * ERROR: loopback failed to start