Use HEALTHCHECK instruction in dockerfile

[LuqiPan]

As brought up by @0xpatrickdev in Agoric/dapp-agoric-basics#24 (comment), currently the consumers of a3p container image would need to implement bespoke script to check the health of a3p container, like in cosgov and dapp-agoric-basics.

It would be great if we could implement the health check in a3p container image itself and then the consumers of a3p container image could use standard docker commands to determine if the container is healthy or not.