[Solution] - Despite the documentation around deleting docker.img not being super clear as to why I would want to delete it, and it being rather counter-intuitive to delete a file to restore a service, I proceeded to follow the instructions to delete it.
It did work though and all my docker containers came back online without having to reinstall them.
I setup a new UNRAID server but couldn't get it to boot off the old USB drive.
I tried to restore a backup of it but it wouldn't boot off that either.
So I created a new USB drive and copied the /config to it. It boots fine, the
/mnt/user/system/docker/docker.img exists but Docker tells me "No Docker containers installed"
How can I get back the Docker containers and their configs?
Appreciate any assistance - Tim