I have some notes from doing it but its been a minute, the overview is:
create your users in and out of the container with the correct ids
edit the conf to pass through the dir and map the ids
edit the subuid and subgid
The documentation on this kinda sucks because its not all in one place so if you find the first link you might get lost without the info in the second. I took me a few forum posts to find out about all the id mapping stuff and finally find the right page.