Add `ping` to Docker image

Users Dgraph
2

You can use “/health HTTP status code 200 and “OK” message if worker is running, HTTP 503 otherwise.”

https://docs.dgraph.io/deploy#more-about-dgraph

And inside the Dgraph image there is a cURL bin if you need to use.

Read more about other restrictions:

https://docs.dgraph.io/deploy#dgraph-administration