execute: fix app containers without root mapping

execute: fix app containers without root mapping

When starting application containers without a mapping for container root are
started, a dummy bind-mount target for lxc-init needs to be created. This will
not always work directly under "/" when e.g. permissions are missing due to the
ownership and/or mode of "/". We can try to work around this by using the
P_tmpdir as defined in POSIX which should usually land us in /tmp where
basically everyone can create files.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>