[FHIR] add readinessProbe to deployment.yaml

[michaelwschroeder]

We need to add a readinessProbe which calls $healthcheck using exec vs. http since we need to grab the user password from an environment variable.

[lmsurpre]

If we needed just the env var's value we could use the kubernetes support for env var interpolation via $( ). Unfortunately, we're trying to use it within an HTTP readiness probe and that doesn't support basic auth except for setting the Authentication header yourself. So we'd actually need to be able to base64 encode the username:password and there is no way to do that from an HTTP readiness probe without writing that password (albeit encoded) right into the deployment yaml.

Instead, the password should be kept in a secret (already done now), and we should use curl to invoke the endpoint.