Proposed fix for issue #772

[necopinus]

Proposed fix for issue #772. To recap:

Right now, enabling dnsmasq within a NextCloudPi Docker container associates your $DOMAIN with the container's private IP address. This means that Docker images cannot use dnsmasq to redirect systems on your home network to your NextCloudPi server.

This change sets the IP address corresponding to your Nextcloud $DOMAIN to the IP (optionally) passed into the docker image (which is stored by 020nextcloud in trusted_domains 6). If trusted_domains 6 is empty or is not an IP address, fall back to the previous behavior.

Tested on my "production" home NextCloudPi server. Note that I only have access to that server and a Chromebook right now, so my ability to test this change more fully is limited.