Fix /dev/null jailing
Fix /dev/null jailing

* Remove unnecessary dir mount of `/dev` which somehow blocked
  write access to `/dev/null` (I tried mounting it read/write too
  but no luck).
* Adds group file
* Also adds a `notroot` fake user

mattgodbolt/gcc-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer