Adding a healthcheck on the Docker Image

[gnock]

Healthcheck on Docker image are great to detect problems as soon as possible.
The process might not crash, but might not respond to external requests (both on the p2p port and rpc port).

Adding the HEALTHCHECK instruction in the Dockerfile would allow a better high availibility.

Possible options are to write a bash script runned each 10s trying to establish a connection on the p2p port, and another instruction making a call to get the blockchain height and expecting a 200 http status.

In the future, a fork detection would also be welcomed.