gentoo-portage/Dockerfile.template: Use 'webrsync --revert'

gentoo-portage/Dockerfile.template: Use 'webrsync --revert'

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.