Update Dockerfile to specifically use CentOS 6

Add support for building this CentOS 6 Docker Image since CentOS 7 Docker Base Images now 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.  Other changes would be needed as well but this makes it work using CentOS 6.