Support RHEL 7 beta

A RHEL 7 beta docker base image can then be created by issuing the
following as root in RHEL 7 beta:

cd contrib
./mkimage-yum.sh rhel7-beta-base

0.8.1

bump_v0.8.0

Merge pull request moby#3098 from creack/bump_v0.7.1

Bump v0.7.1