Problems following https://appwrite.io/blog/post/how-to-back-up-your-appwrite-data

To me it is unclear what the author means when he writes "Please note that the Appwrite instance should be shut down". I am using docker. What exactly am I supposed to shut down to avoid "service "mariadb" is not running" or "docker: Error response from daemon: volumes-from specification cannot be an empty string."

That's for docker volumes, not for MariaDB

I used
docker compose stop appwrite
which let me at least run the MariaDB part of the backup.

But for the Volumes I am at a loss. Which part of appwrite am I suppoed to stop?

You don't need to stop anything for running:

For the volumes, you should stop the entire stack if possible

Do you mean something like "docker compose stop"? As far as I know, that is not possible. This is the prompt:
mkdir -p backup && docker run --rm --volumes-from "$(docker-compose ps -q appwrite)" -v $PWD/backup:/backup ubuntu bash -c "cd /storage/functions && tar cvf /backup/functions.tar ."

The part "docker-compose ps -q appwrite" generates an empty String if appwrite is not running resulting in the prompt not being executed.