Configure CI container to be able to build images on the CI server

Adds two build args (RVM_GID and DOCKER_GID) to the centos:ci image, so when the image is built we can match the CI server's docker and rvm group's GIDs with the corresponding group on the image. This prevents permissions errors from cropping up with trying to run docker commands when binding docker.sock as well as any rvm issues that could come up.