Add `ping` to Docker image

I think you are misinterpreting what I would like to do. I essentially don’t want to start the server until the initial sync occurs. I’m not checking if the Dgraph server is up, I know it isn’t because I haven’t started it yet. Checking the /health endpoint doesn’t do anything for me unless I know what “200 OK” means.

The description is very vague. Does it mean the data has loaded? Does it just mean the zero server isn’t broken?