portage: Add a Portage-volume-exporting image

This builds the Portage snapshot into a stand-alone volume, which can
be mounted using -volumes-from.  There's currently no
Dockerfile-supported way to mount this for builds, so gentoo-portage
still goes the emerge-webrsync route.

We need something to fill the role of /bin/sh in the Portage-snapshot
container, so I'm using the /bin/busybox from the 'gentoo' image.  As
of 2013-12-12, it's:

  sys-apps/busybox-1.21.0
    USE="ipv6 pam static -livecd -make-symlinks -math -mdev -savedconfig
         (-selinux) -sep-usr -syslog -systemd"