support IPv6

[rriemann]

Dear devs,

according to this Mastodon post https://mastodon.social/@mxmehl/108248276033982936 , the people from the Free Software Foundation could not employ this project for their purposes because of unsupported IPv6 routing in the Docker internal networks.

If this is supported with cadd-gen, it would be cool if someone could document a minimal working example! If not, maybe we can report this issue upstream at Docker or change caddy-gen if necessary. :)

Their solution was to develop a similar tool:

https://mehl.mx/blog/2022/docker2caddy-an-automatic-reverse-proxy-for-docker-containers/