Add CACHE_DIR to allow moving cache location

[akiller]

Currently the model cache file is written to the location of the MODEL_DIR environment variable if set.

I want to deploy the label studio ML backend to an Azure container app. My model is persisted to an Azure storage file share mounted at /app/models and so my MODEL_DIR is set to /app/models.

But Azure files don't work with Sqlite database, and it will error with Error: SQLITE_BUSY: database is locked

I want to add a new CACHE_DIR environment variable which will allow the cache to be moved to an alternative location inside the container which should fix my problem.

If not set, it will fallback to using MODEL_DIR so it shouldn't cause any problems for existing users.

I haven't tested this (sorry!).

[akiller]

I haven't tested this (sorry!).

I have now tried this on Azure and it does work with this fix.