For each service I run, I have a volume for the persistent storage. For example: Nextcloud has a maria (mysql) db and a server. In my /opt/nextcloud I have a script for creating/updating the docker images. Persistent data lives in /opt/nextcloud/srv/{db,data}. I used systemd for management/dependencies (I don't use docker compose, just simple, single instances that are easy to recreate).
With this setup, how do you deal with data backups, especially if multiple of your docker instances have databases of their own?