Add dind test to validate docker build

[JoshuaC215]

• Add an optional "fake" model which always returns the same answer
• Add a /health check endpoint to the service
• Add an integration test that assumes the service is running in docker
• Add a step in the test-docker.yml workflow to actually run the containers using dind, confirm the health checks pass, and run the integration test on the service (note: it doesn't actually execute the streamlit app to confirm it loads)