I think you overlooked this line:
DOCKER_IMAGE_TYPE="folder"
I don't have a docker.img
In the above location are the files and folders that sit in your case in the docker.img but in my case without a image, they are stored directly in this path.
Yes sure this is the namig scheme from the ZFS documentation but maybe something on Unraid doesn't like that or Docker... Whatever the case may be, at least you can try it.
As said, I run Docker and everything from my ZFS Mirror without a issue so far.
Yes, but if I'm not mistaken this is the recommended way to shutdown Unraid, but I think that there are many ways to shutdown Unraid.