Update Docker healthcheck to include result handlers

[cicdw]

There are certain situations that we can catch prior to deployment that would result in a Flow not behaving as expected. In particular, if a user tries to deploy a Flow which:

• has a task with retry settings
• that task also has a keyed upstream edge
• the upstream task has no result handler

Then retries will fail to work in Cloud. Because this situation can be inferred apriori we should raise an informative error during the Docker health check.

Similarly, if a user has tasks with checkpoint=True that don't have corresponding result handlers, we should raise an error.

Note that if the Flow itself has a result handler (but the Task does not), then all of this is OK because the Flow's should be the fallback.