[Request]: improve builder storage configuration and management

### Feature or enhancement request details

We should tune the builder storage configuration so that is uses a smaller volume than 512GB. This represents a considerable amount of storage on many users' systems.

Along with the smaller volume, we check to see that the buildkit cache management policy we're using is sane, and we should add some means by which `buildctl prune` gets run enough so that the builder doesn't run out of storage.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct