Do a short recap on Docker/Containers/Images/build/pull/run. Repositories, Registries, Index.
Then looked at:
Can build a quick Fedora/HTTP Image from Dockerfile:
It is sometimes necessary to have files in a container that shouldn't ever end up in an image. These files are generally some form of private key or password that aren't allowed to be distributed. This document details a few usecases for such files and their requirements.