[Chore] [Docs] List all possible environment variables

[LeoColman]

Hi!

With an exception to this point in the README:
https://github.com/lucaslorentz/caddy-docker-proxy?tab=readme-ov-file#caddy-cli

There's not a lot of mentions to the Environment Variables.

For instance, if I want to know the default value of CADDY_INGRESS_NETWORKS I need to track it down all the way to

caddy-docker-proxy/cmd.go

Line 217 in 400e904

if ingressNetworksEnv := os.Getenv("CADDY_INGRESS_NETWORKS"); ingressNetworksEnv != "" {

to see that it doesn't have a default value.

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It would be great if instead I could see it formatted in a special documentation page

Variable	Description	Default Value
CADDY_INGRESS_NETWORKS	Which network should be considered in docker for ingress	networks attached to controller container