allow running registry with in-memory database

[maorfr]

Is your feature request related to a problem? Please describe.
as a platform engineer, i want to use a git repository to maintain the registry's content.

Describe the solution you'd like
an option to run the registry and specify to use the in-memory database

Describe alternatives you've considered
running a small mongodb container as is and ignore it

Additional context
happy to work on this issue if it makes sense!
i believe that for internal registries, a git workflow solves a lot of problems.