bubblewrap

It's a tradeoff. It's certainly better to give untrusted users access to unprivileged user namespaces than to give them access to /var/run/docker.sock, which straightforwardly gives them full root. Another project in this space is bubblewrap https://github.com/containers/bubblewrap , which can run either with unprivileged user namespaces or by being setuid. It's intended to create an environment for container runtimes to use so that the container software itself doesn't need to be privileged, and the idea is that bubblewrap itself uses the privileged interfaces to set up the environment and then drops privileges before running user-provided code, so it shouldn't introduce more risk.

Bubblewrap (https://github.com/containers/bubblewrap) on Wayland. You might have to make a chroot sort of environment for it for it to be convenient enough to use in practice unless you’re okay with exposing the (read-only) contents of /usr, for example, to the sandbox.