Feature request: add HEALTHCHECK

[bkuhl]

Docker supports defining a command to determine if the container is running as intended (healthy), broken for some reason (unhealthy) or still initializing (starting):

• https://docs.docker.com/engine/reference/builder/#healthcheck

Adding this to this image with the semantic "the mysql server is online and functional" would be quite nice.

Since the last HEALTHCHECK defined by a Dockerfile is the one actually used, images based on this one could define their own healthcheck without "interference" by the one provided by the mysql image.

Related to docker-library/postgres#282 and redis/docker-library-redis#91 indirectly.

[yosifkit]

Closing as duplicate of #196.