The short version. 99% of the code has been written and merged already.
The reason the question of running a distribution in a container even comes up is that people are already doing that today.
At the level of a distribution there are still a few rough patches in the coverage so it was worth having the discussion. The big piece left in my opinion is finishing the user namespace and that work at this point is a few small patches and review review review. It is the permission checks we are touching and it is important to look very carefully so you don't fat finger something.