The proot mode was broken from the start because in contrast to
fakechroot, no ownership information can be retained across multiple
invocations of proot. Since mmdebstrap started using apt from the
outside by setting DPkg::Chroot-Directory in mmdebstrap 0.8.0 proot mode
was finally completely broken because proot cannot wrap the chroot call
done by apt. Users of proot are recommended to run mmdebstrap in
fakechroot mode and then use proot with the resulting directory.