Private registry base OS best practice

I have to operate on a disconnected network. We setup a private docker registry fine. Moved various images over. But the issue is when you try to build a new image on the disconnected network when you do something like a yum install (lets use Centos as an example) it looks to the internet for those files. Not a huge big deal you just change CentOS-Base.repo to look at the updated OS repositories we have on our disconnected network and it will work.

Issue is what is the best practice to do this? Should I create a new base Centos image that has the new CentOS-Base.repo pointing to my internal OS update repositories or should I make it an argument into the dockerfile where you pass a argument in on the build to copy in the correct version of CentOS-Base.repo fro the environment? Or is there some better way to move base OS images between various environments so you don’t have to change anything?