Seeing up NodeBB with docker isn't officially supported. I suggest you use the Ubuntu guide: https://docs.nodebb.org/installing/os/ubuntu/
unconfigured domain pointed to my server ip opens duplicate forum
I don't know if I get your question right. I think you mean that you use apache as load balancer, and that all other domains that set an A record to your server do get your own forum (not a duplicate, but the actual copy).
This makes me think you added the configurations in the default apache configuration. You should add the configurations to a seperate file, where you set your server name only to the domain where you want the forums to be. The default configuration should not be changed.