Health check support

[crummy]

Problem

I want to know when the initial sync is completed, so I can safely boot up dependent containers that rely on these files.

Solution

Add health check support:

• When initial sync is complete, write a file to the container's file system
• Point the health check configuration to check for the existence of this file
• During backups, if a backup fails, remove the file (this one isn't essential for my needs but seems part of a health check feature)

[tsmith023]

@fabiob, is there any chance this could be merged? It would be a great change that makes spinning up standalone docker compose environment much easier. Cheers!

[crummy]

@tsmith023 in a pinch you can use the image I built here: https://hub.docker.com/r/crummy/s3-volume