pidns reaper requirement

[JosiahWhite]

Hi, it is unclear to me why the reaper requires a pidns to function. It seems like it would be possible to still reap child processes via waitpid? Unless I am missing something?

This requirement prevents rootlesskit from cleaning up zombie processes in the context of buildkit if a build is killed and buildkit is running in rootless mode.

More information is available here: https://github.com/moby/buildkit/issues/2855