use f21 image from fedora rel-eng

More info about this image can be found here:
http://koji.fedoraproject.org/koji/taskinfo?taskID=7978938

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

	modified:   Dockerfile
	new file:   fedora-21-beta.tar.xz
	new file:   fedora-docker-base-1dddaee.ks