s3proxy has a filesystem backend [0].

Possibly of interest: s3gw[1] is a modified version of ceph's radosgw that allows it to run standalone. It's geared towards kubernetes (notably part of Rancher's storage solution), but should work as a standalone container.

[0] https://github.com/gaul/s3proxy [1] https://github.com/s3gw-tech/s3gw