ppp0 will disappear from the host namespace. You can't share one IP address with a container and not your other IP addresses. (Apart from doing NAT).
It looks like this might require a very recent kernel though. http://www.spinics.net/lists/netdev/msg339236.html
OR (machine is created after ppp0):
ip link set dev ppp0 netns $PID
where $PID is the pid in the host namespace of a process in container, obtained using
P=$(machinectl $MACHINE_NAME show --property=Leader)
$MACHINE_NAME is the value passed to the
-M option of