You can use any user to run the container. It should not be "root" and the user must have access to the media files (in case of Plex).
No need to pull the image in Portainer manually. If you run the docker-compose file in Portainer the image will be pulled automatically.
This is why I keep getting confused. You say the user must have access to the media files but I had a thread before asking how the users are actually used. There doesn't seem to be anything that uses the usernames on each computer. I have no idea what "user" my laptop is considered but it can access all of my files just fine. My desktop cannot though (both are running Windows 10). On my desktop I can open a browser, navigate to 192.168.1.3 and log into OMV with any of the accounts I have set up, but when I try to actually open the folder in the network tab to view my shared files it says I don't have access.
Also, at what point exactly does the Plex image get pulled automatically now? From the OMV screen I click OMV Extras > Docker Tab > Portainer Dropdown and click Open Web > Log in > Local. From here it doesn't matter where I go (images or container>add container) but I don't get anything to give me a Plex image. The only thing that shows up in the window is portainer/portainer:latest (like in the first post). When I first made this thread I went straight to the container tab to try and set everything up and people said I had to go to the image tab first to pull the Plex image. That worked then but it's not working now so I have no idea what step I'm missing.