Update Dockerfile to specifically use CentOS 6

Add support for building this CentOS 6 Docker Image since CentOS 7 Docker Base Images exist.  This dockerfile would try to build a CentOS 7 machine with the incompatible EPEL 6.  RedHat/CentOS should be using the new EPEL 7 Repository.
Merge pull request #1 from ddcruver/master

Update Dockerfile to specifically use CentOS 6