/usr/lib/rootstrap/modules/umount is in rootstrap 0.3.25-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/sh -e
# clean up anything left behind
umount -n $TARGET/proc || true
umount -n $TARGET/sys || true
umount -n $TARGET
|