Feature request: Override detected LAN, or add additional networks for inbound firewall rules

[JacksonWrath]

What's the feature 🧐

An environment variable to override the LAN network, or at least add additional networks for inbound connections, would be useful for cases where Gluetun isn't able to detect the actual local network.

Extra information and references

Why is the local network not detected for me?

I'm running this in Kubernetes, with Calico as the network plugin. It turns out Calico sets the default route to 169.254.1.1, so Gluetun isn't seeing the actual pod network.

I got around this with custom iptables rules, which works, but would've been convenient to not have to do that.

I know Gluetun is primarily targeted at Docker setups, and my issue was a pretty specific edge-case, so fair enough if this is out-of-scope.

[github-actions]

@qdm12 is more or less the only maintainer of this project and works on it in his free time.
Please:

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  @qdm12 usually checks issues at least once a week, if this is a new urgent bug,
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