CAS registry server is stateful?

Thank you for this great bazel rule.

I was looking at using the CAS registry based container upload. The docs are not completely but what is the storage for the `rules_img_tool/cmd/registry` server? Based on my observation the container metadata is stored in the registry server. 

Is there any way to persist these metadata files on disk or some durable storage? Or it is just in-memory cache?