See the Docker homepage and the Docker repository for more information about Docker.
In a nutshell:
Docker is an open-source project to easily create lightweight, portable, self-sufficient containers from any application.
Latest stable is in the portage tree proper under the name app-emulation/docker. If you just want the latest release version of Docker, that's the best place to find it.
Be sure to check out the Gentoo Linux installation instructions at docker.io for important notes and direction regarding the Gentoo ebuilds for Docker.
If you want a -bin ebuild, the latest and greatest changes before they're pushed to tree, or a live ebuild, you're in the correct place.
This repository is listed with layman. Ensure that you have layman (emerge -av app-portage/layman), and invoke the following:
layman -a docker
After adding this overlay, app-emulation/docker should be available for emerge (or alternatively, app-emulation/docker-bin):
emerge -av app-emulation/docker
Note also the existence of dev-util/gordon (see https://github.com/docker/gordon for more details).