This create a base image that can be used in build environments with different requirements.
Docker projects (other than this) shall build both in a standard environment with a direct internet connection and in a restricted environment with proxy access to whitelisted repositories.
Create and maintain an image tag that will point to either the upstream centos:7 (i.e., docker.io/library/centos:7), or to a local image that has modified URLs for YUM repos etc.