[geofft]

I wish Linux would do this. Patches are available: https://lwn.net/Articles/849125/

Yes, you can do this on Linux with a user namespace, but a user namespace changes the view of user accounts. You have to map every usable UID inside the namespace to a UID you control outside the namespace. At best, you can map a range of UIDs you control to "real" users (root, 1000, etc.) inside the namespace, but they won't be real users outside the namespace. If you're on a multi-user system, seeing other people's files as owned by "nobody" is confusing.

It should be enough to use NO_NEW_PRIVS mode, meaning setuid transitions are not allowed. Then it doesn't matter what user IDs you see inside the chroot.

In fact, back when Linux introduced the NO_NEW_PRIVS flag (almost a decade ago!), this was one of the motivating use cases.