Support chroot containers

[nponeccop]

PRoot has an advantage that it doesn't require root privileges. But at the same time it's disadvantage is that groups/users aren't supported (see #141 for example)

So for those who want to run containers on old kernels without proper Docker support, chroot provides a better alternative than proot. So it's a nice feature to have

[TangYuanMaster]

Can I run udocker on chroot?