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

ostree/dockerfile,fr34k8/dockerfile,wking/dockerfile