We want to be able to reach the external PDN from the PDP contexts,
and hence need to enable IPV6 forwarding between the tun device
and the PDN device(s). The latter are added using "docker network
connect"
For some reason, it appears that if docker sends SIGTERM the default
action (kernel auto terminate) is not installed, but it expects the
program to handle it directly. Let's tell docker to use SIGINT as
this is what ggsn always handled.