Add systemd-docker ebuild

cros-workin is a special wrapper around the "git-2" eclass that allows you to pull the source directly from the local source tree checked out by repo. If you don't need that functionality you can instead download release tarballs or snapshot tarballs which are easy to mirror. It basically just involves filling out SRC_URI correctly.

Release tarballs:
https://github.com/coreos/portage-stable/blob/master/app-arch/lbzip2/lbzip2-2.2-r1.ebuild

Snapshot tarballs using the vcs-snapshot eclass:
https://github.com/coreos/coreos-overlay/blob/master/sys-apps/efunctions/efunctions-1.ebuild