I spent two hours today trying to figure out why Nextcloud couldn’t read my data directory. Docker wasn’t mounting my data directory. Moved everything into my data directory. Docker couldn’t even see the configuration file.
Turns out the Docker Snap package only has access to files under the /home directory.
Moral of the story: never trust a Snap package.
Does it clearly need access to files outside the /home directory though?
You said your volume mount failed. How about mounting something inside your home folder into the docker container?