gentoo-portage/Dockerfile.template: Set rc_sys="lxc"

gentoo-portage/Dockerfile.template: Set rc_sys="lxc"

Avoid:

  $ docker run -p 8010:8010 wking/buildbot
  ...
   * Caching service dependencies ...                             [ ok ]
   * You are attempting to run an openrc service on a
   * system which openrc did not boot.
   * You may be inside a chroot or you may have used
   * another initialization system to boot this system.
   * In this situation, you will get unpredictable results!
   * If you really want to do this, issue the following command:
   * touch /run/openrc/softlevel
   * ERROR: syslog-ng failed to start
  ...