1) " I assume that any folders that will be accessed by Docker have the correct permissions. Finally, I assume you'll forward any necessary ports in your router. " I need to do this - how, in easy steps?
You don't have to do anything with the ports on your router (unless you want to access from outside). Don't do anything on the router now.
2) I've created the User " specifically for Docker. " - What rights do I need to give - given question 1 above?
If you created the user "specifically for docker" in the GUI of OMV he already member of the users group. That should be enough. We will confirm later.
3) " SSH into your OMV system and run the command: " I'm not at all sure how to do this - but I tried logging in to OMV with my server IP addresss/shellinabox and got a login prompt - whatever user I tried to enter I just got the password prompt repeating - and I know it was the correct password each time. I need this step to find the UID and GID. How exactly do you "SSH in"
Did you use the user "root"? That should work for SSH. Please write if you used root and it did not work.