gentoo-portage/Dockerfile.template: Remove 'emerge --sync' call

specing points out that the rsync behind 'emerge --sync' is vulnerable
to man-in-the-middle attacks.  In any event, it's probably more
reproducible to stick to the daily snapshot releases used by webrsync
and ignore any changes that have since landed in the rsync mirrors.
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.