Is Extras just another reference to the Fedora EPEL Project? I can't find a reference to RedHat Extras anywhere except on a press release, and the one developer comment (on a quick Google search) that mentions anything about extra packages points back to the Fedora EPEL site.
Also, docker-io and docker-registry are located in the EPEL 7 beta repository -
http://dl.fedoraproject.org/pub/epel/beta/7/SRPMS/repoview/letter_d.group.html