This doesn't really answer the question. Yes, the Linux API seems more flexible, but when you think about it, it really isn't, because all the models that actually make any sense can be implemented using simpler interface, which is what jails provide.

One real difference is that you need to be root to create a jail. It'll get fixed eventually - FreeBSD already has unprivileged chroot, jail isn't that much different.

Yes, I am aware that it's got more moving parts. What are you using this flexibility for?

And how is it mixing and matching these APIs? Given that there's an OCI-compatible runner for jails (runj, compatible with runc -- which is what docker uses to start containers), it seems to me that Docker isn't in actually using the flexibility afforded by the APIs here, but is just using a relatively fixed set of options.

If I'm wrong: what is it using, and what problems is this flexibility solving?