This adds an additional network to the traefik container with a public IPv6 network attached to it. Docker will automatically assign a free address to the traefik container from the IPv6 subnet
Internally traefik will still forward traefik over an IPv4 network
Note that, because docker does not do any NAT for IPv6 or proxying, and this uses publically routable IPv6 addresses, traffic directly reaches the containers, unless you block it with a firewall. For 80/443, you actually want this to happen though.