To grab a specific snapshot of the Portage tree. Otherwise the
snapshot you get depends on when you run webrsync. The public mirrors
only seem to keep about a week of snapshots, so you may want to use
something like package-cache to cache interesting snapshots locally.
#RUN echo 'SYNC="rsync://rsync.us.gentoo.org"' >> /etc/portage/make.conf
RUN mkdir -p /usr/portage
RUN chown -R portage:portage /usr/portage
-RUN emerge-webrsync
+RUN emerge-webrsync --revert=${TAG}
RUN eselect news read new