gentoo-layman/Dockerfile.template: Update for modern layman

gentoo-layman/Dockerfile.template: Update for modern layman

From [1]:

  This method uses the new portage plugins system.  Layman will create
  its configuration file in /etc/portage/repos.conf/ directory.  There
  is no need for PORTDIR_OVERLAY changes in /etc/portage/make.conf.

  Configure layman to use repos.conf method in /etc/layman/layman.cfg.
  New installations of layman will probably have this already set
  correctly:

  ...

  If you have >=layman-2.3.0 installed, you can force a rebuild of
  layman's repos.conf files:

    root # layman-updater -R

although I've gone with the long option --rebuild for clarity in this
commit.

Update the fetch command because:

  $ layman --version
  2.4.1
  $ layman --help | grep -A2 -- --fetch
    -f, --fetch   Fetch a remote list of overlays. This option is
                  deprecated. The fetch operation will be performed by
                  default when you run sync, sync-all, or list.

[1]: https://wiki.gentoo.org/wiki/Layman#repos.conf_method_.28default.29