It matters only if “the docker hosts external IP” your dns resolves is a public IP. In that case packets travel to the router which needs to map/send them back to the docker hosts LAN IP (NAT-Reflection). With cgnat this would need to be enabled on the carrier side, where you set up the port forwarding. If that’s not possible, split-DNS may be an alternative.

If “the docker hosts external IP” is actually your docker hosts LAN IP, all of that is irrelevant. Split-DNS would accomplish that.

Are you hosting behind NAT / at home? If so, you may need to enable NAT reflection on your router.

It depends on your usage. If you are downloading hundreds of GB per month or more, a block account does not make sense.

Personally I get almost everything off torrents, so I also have some Block accounts which last me many years for the occassional use.