Add general health endpoint

[leighmcculloch]

What problem does your feature solve?

It's not obvious to new users how to tell the image is fully ready to be used. Being ready is some combination of multiple services being ready.

Folks end up writing scripts to test it in different ways.

What would you like to see?

A /health endpoint that reports a HTTP 200 if the image is fully ready for use.

What alternatives are there?

Do nothing.

cc @stellar/devx

[github-actions]

This issue is stale because it has been open for 90 days with no activity.