Thanks for your comment.
There is only one user, tux, which is uid 1002 and tux does not exist in the host's /etc/passwd.
The users do not have root access in the containers.
Cgroups is used to impose limits on RAM, number of processes, visible CPUs, etc.
I made this site in evenings and weekends, and have put it up for the public to use. I want find out sooner, rather than later, if the whole idea is flawed.