That's actually one of the primary use cases at the moment. The way Mutagen works with Docker is by using `docker cp` to inject one of its "agent" binaries into the container and then communicating with that binary over `docker exec`.
You can also orchestrate your setup a bit (e.g. to work with Docker Compose) using Mutagen's new orchestration infrastructure. You can find an example of this at the bottom of the Mutagen homepage: https://mutagen.io
It's designed for a different purpose - bidirectional sync. Though it does use the rsync algorithm for efficient transfers. It also does file monitoring. Closer to Unison really, though with a slightly different and more modern approach.
RestKit (http://restkit.org/) is working on something similar. Right now you have to manually push your data back to the REST service, but in version 0.11.0, they are going to add exactly the kind of functionality you're looking for:
Hey HN,
Got tired of waiting for Apple to add this feature, so I decided to try to implement it myself. This is my first attempt at Cocoa hacking, so please feel free to give me suggestions if I did something wonky. Also, the plugin is open-source, so please feel free to check it out for yourself.
You can also orchestrate your setup a bit (e.g. to work with Docker Compose) using Mutagen's new orchestration infrastructure. You can find an example of this at the bottom of the Mutagen homepage: https://mutagen.io