[3.16] rootbld in production use

This is not a formal system change proposal (yet). I would like to gather information about how to implement rootbld on the builders so that we can switch to rootbld in production during the 3.16 development cycle, thus ensuring each build happens in its own dedicated container, which avoids a lot of problems we have today.

I have time dedicated to work on this issue, I just need to know what areas need addressing.