gentoo-portage/Dockerfile.template: chown portage:portage /usr/portage

When the usersync feature is enabled (which it is by default),
emerge-webrsync uses the owner and group of /usr/portage
(${repo_location}) for the unpacked tree.  This commit avoids having
the whole tree be owned by root:root.
README.md: Remove 'is mounted from the host' typo

This was leftover from an old writeup of a host-mounted workaround,
and should never have been committed.
build.sh: Bump DATE to 20140206

Move past the recent Python-and-/dev/shm issues now that Catalyst is
fixed.  I've built the full suite of images with 20140206 without
issues.