Use ipv6nat container to implement IPv6 functionality

NAT! On IPv6? Who hurt you? Okay, this isn't a permanent solution but right now Docker IPv6 support is half-baked and it's difficult to use and I don't like that. So for now, we essentially use a like for like IPv6 implementation of the way Docker networking works for IPv4.

Details here: https://github.com/robbertkl/docker-ipv6nat#why-would-i-need-this.