pkgsrc can install a full UNIX system inside another system in any location, as (I think) any user that can access that location.
It might be more “practical” than Nix, and might be interesting for deploying software with complex dependencies? (Without using something like Docker.)
There are more interesting things, just one example:
Best of all, the binaries in /home/ourapp/root can reasonably be expected to run on any version of Linux available in the past 15 years: simply extract a tarball of the build to the target machine.