If an app needs a database, it gets a database server container, instead of getting a user and database on a shared database server as things used to be done. Every little django app has its own postgres container. Every wordpress site gets its own mysql container. That is the modern way.

Those database containers get a PVC/volume/mount for their data dirs. The only thing ever connecting to them is their "owner" application container. So at that point, why not drop the postgres container and PVC mount a sqlite directory in the app container? The result is the same.