What does it mean "Docker is the way to go"? Is Docker a replacement for the Repo in OMV-Extras?
Did you click on the link? It shows you how to setup docker and plex using the docker plugin from omv-extras. Docker is a replacement for some of the plugins not the omv-extras repo.
Can I use my "original" Plex database anyhow when moving to the Docker image installation for Plex (new)?
I'm sure you can but I don't use plex. So, someone else will have to answer that.