Docker image for `ilp-node` needs a HEALTHCHECK

[aphelionz]

Due to route propagation delays, sometimes the container can appear spun up while there are still connectivity issues. It would be good if the image could report out a health check once everything is stable. This will help with k8s deployments, among other things.

[koivunej]

In testing code, this is currently done with repeatedly issuing GET requests, for example in examples/simple/README.md (cannot link directly): wait_to_serve "http://localhost:7700" 10.

Similarly in ilp-node benchmarks, which just re-attempts the payment until it first succeeds.

Marking this as good-first-issue as only docker knowledge is required, assuming the GET / request is enough. It would be forwards at least.