Persist locale in the image

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.

Containerfile

 ARG STAGE0_TAG
 FROM nest/stage0:${STAGE0_TAG}
 
+ARG FACTER_build=stage1
 ARG FACTER_emerge_default_opts
 ARG FACTER_makeopts
 ARG FEATURES
 ARG PROFILE
 
+# Locale is managed by Puppet, but
+# it also needs to be persisted in the image.
+ENV LANG=en_US.UTF-8
+
 RUN eix-sync -q
 
 # Set up some conveniences
-RUN FACTER_build=stage1 puppet agent --test --certname ci --environment main --tags nest::base::distcc,nest::base::portage,nest::base::puppet --logdir /var/log/puppet --rundir /var/run/puppet --vardir /var/lib/puppet --runtimeout 0; [ $? -eq 2 ]
+RUN puppet agent --test --certname ci --environment main --tags nest::base::distcc,nest::base::portage,nest::base::puppet --logdir /var/log/puppet --rundir /var/run/puppet --vardir /var/lib/puppet --runtimeout 0; [ $? -eq 2 ]
 
 # Switch to desired profile and make the base system consistent with it,
 # breaking out of a possible circular dependency between mesa and libva.
@@ -20,7 +25,7 @@ RUN emerge -v --update --deep --newuse --with-bdeps=y @world
 RUN emerge --depclean
 
 # Configure the stage
-RUN FACTER_build=stage1 puppet agent --test --runtimeout 0; [ $? -eq 2 ]
+RUN puppet agent --test --runtimeout 0; [ $? -eq 2 ]
 
 RUN rm -rf /var/db/repos