Allow configuration of appdata directory location

I'm using Nextcloud 14.0.4 via the docker image. On my host machine, I mount a CIFS share to my filesystem, and I use this mounted directory as the data directory for Nextcloud. For performance reasons, my expectation was that Nextcloud would use the data directory as little as possible to avoid IO contention. Basically, it should only ever be used when an actual backed up file is accessed (actual files they sync). After all, the only reason I mount this way is so that the main storage for users goes to my NAS.

However, I'm observing that there's an appdata_abcdefg directory there, which has a lot of metadata in it. The most concerning piece of data in there are the image previews. I'd like a way to reduce the amount of usage that Nextcloud puts on the data directory. I'd like it to only store user data files, and the rest should reside on the local filesystem.

Is there a way to do this? How can I prevent metadata from going to my NAS for performance reasons?

The issue names no files, tests, or entry points. Start by reviewing how Nextcloud 14.0.4 places appdata metadata and image previews under the configured data directory, then determine the supported configuration boundary. Done means users can keep user files on the mounted data directory while relocating metadata and previews to local storage.