Let Puppet manage make.conf

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.

.gitlab-ci.yml

 ---
-variables:
-  JOBS: 8
-
 default:
-  image: '${CI_REGISTRY}/nest/tools/buildah:haswell'
+  image: '${CI_REGISTRY}/nest/tools/buildah:${CI_HOST_PLATFORM}'
   before_script:
     - 'buildah login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"'
 
@@ -15,9 +12,6 @@ default:
     - 'buildah bud --pull
                    --security-opt seccomp=unconfined
                    --build-arg STAGE0_TAG=haswell
-                   --build-arg FACTER_build=stage1
-                   --build-arg FACTER_emerge_default_opts="--jobs=${JOBS} --usepkg"
-                   --build-arg FACTER_makeopts="-j${JOBS}"
                    -v "${PUPPET_CERTIFICATE}:/etc/puppetlabs/puppet/ssl/certs/ci.pem"
                    -v "${PUPPET_PRIVATE_KEY}:/etc/puppetlabs/puppet/ssl/private_keys/ci.pem"
                    -v "${PWD}/debug:/usr/lib/debug"

Containerfile

 ARG STAGE0_TAG
 FROM nest/stage0:${STAGE0_TAG}
 
-ARG FACTER_build
-ARG FACTER_emerge_default_opts
-ARG FACTER_makeopts
-
 RUN eix-sync -q
-RUN puppet agent --test --certname ci --logdir /var/log/puppet --rundir /var/run/puppet --vardir /var/lib/puppet --runtimeout 0; [ $? -eq 2 ]
+RUN FACTER_build=stage1 puppet agent --test --certname ci --logdir /var/log/puppet --rundir /var/run/puppet --vardir /var/lib/puppet --runtimeout 0; [ $? -eq 2 ]
 RUN rm -rf /var/db/repos
 
 CMD ["/bin/zsh"]