Support IPv6-only containers

[herver]

Hello there,

First: thank you very much for the project, it is really useful !

I have a personal VPS that only has native IPv6 connectivity and I'm running a few containers with Podman (which works very well with caddy-docker-proxy) in an IPv6-only container "network".

I noticed that caddy-docker-proxy only exposes containers properly if they have an IPv4 address (eg: the IPAddress field of podman inspect output). Would it be possible to also expose containers that only have the GlobalIPv6Address property (aka IPv6 connectivity), maybe this could be opt-in with a feature flag or an environment variable ?

Here's an example of an IPv6-only container on a specific web network:

 "Networks": {
      "web": {
           "EndpointID": "",
           "Gateway": "10.89.0.1",
           "IPAddress": "10.89.0.28",
           "IPPrefixLen": 16,
           "IPv6Gateway": "fdcd:e835:833f:42::1",
           "GlobalIPv6Address": "fdcd:e835:833f:42::2f",
           "GlobalIPv6PrefixLen": 112,
           "MacAddress": "52:96:65:7a:20:ed",
           "NetworkID": "web",
           "DriverOpts": null,
           "IPAMConfig": null,
           "Links": null,
           "Aliases": [
                "freshrss",
                "82b4e3a48676"
           ]
      }
 }