I see there's already an ebuild here: https://github.com/tianon/docker-overlay/blob/master/app-emulation/docker/docker-1.11.0.ebuild
I've been looking at this today. We need a newer version of app-emulation/runc than we have in the tree; I'm checking with the maintainer to see if we can bump it.
Docker 1.11.0 is now in the tree.