GitLab

Make FACTER_build a build argument.  It looks nicer and it doesn't hurt
to think of it as just another parameter to the build.

See also: stage0@9f790786

Parameterize the GitLab CI script and add the profile change to the
Containerfile.

Puppet does weird calculations with distcc that don't work when the CI
host is also a distcc server.

There is no need to pass build options from the CI host because the
algorithm Puppet uses to make /etc/portage/make.conf works just as well
inside the container.