Você não está obtendo nenhum roteamento / trabalho IPv6 dentro dos contêineres porque, como vimos, não há endereçamento IPv6 funcionando dentro deles, mas apenas no nível do host.
De acordo com o guia do usuário do IPv6 - rede - IPv6 com o Docker
By default, the Docker server configures the container network for IPv4 only. You can enable IPv4/IPv6 dualstack support by running the Docker daemon with the --ipv6 flag. Docker will set up the bridge docker0 with the IPv6 link-local address fe80::1.
By default, containers that are created will only get a link-local IPv6 address. To assign globally routable IPv6 addresses to your containers you have to specify an IPv6 subnet to pick the addresses from. Set the IPv6 subnet via the --fixed-cidr-v6 parameter when starting Docker daemon:
dockerd --ipv6 --fixed-cidr-v6="2001:db8:1::/64"
The subnet for Docker containers should at least have a size of /80. This way an IPv6 address can end with the container’s MAC address and you prevent NDP neighbor cache invalidation issues in the Docker layer.
With the --fixed-cidr-v6 parameter set Docker will add a new route to the routing table. Further IPv6 routing will be enabled