[FEAT] Local IP option

[gabrielecalvo]

Is this a new feature request?

• I have searched the existing issues

Wanted change

Have the ability to set the internal ip instead of the external one.

Reason for change

For some home-lab cases using dns certificates it would be nice to be able to set the local ip instead of the external ip.

Proposed code change

Something like UPDATE_IP=local-ipv4 could do the trick.
Maybe just adding the following extra elif-branch in https://github.com/linuxserver/docker-duckdns/blob/master/root/app/duck.sh

elif [ "${UPDATE_IP}" = "local-ipv4" ]; then
        echo "Detecting local IPv4 via hostname command"
        IPV4=$(hostname -I)
        RESPONSE=$(curl -sS --max-time 60 "https://www.duckdns.org/update?domains=${SUBDOMAINS}&token=${TOKEN}&ip=${IPV4}")

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[j0nnymoe]

I feel this is something that doesn't need to be automated and you could do the same by just manually setting your duckdns address to a local IP. Or if you're needing both internal and external, you'd use duckdns.