/etc/mtab doesn’t exist after bootstrapping a debian container, and will
be created as regular file after first start.

That leads to at least two errors:
- output of `mount` is wrong and get messed up the more often you
  start/stop the container
- /dev/pts/ptmx has wrong permissions
Signed-off-by: Holger Amann <holger@sauspiel.de>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>