Health check to indicate volume is ready

[crummy]

I'd like to know when my container is done booting up (i.e. has pulled all the files in my s3 bucket). I'm imagining a health check I could use in my docker-compose file. Is there something I can use for this?

(Thanks for the project! This is really a lifesaver for me.)

[crummy]

I've opened a PR to add support for this: #18