configuring Traefik healthchecks from app config

[bastianh]

Description of feature

Labels should be added to the web containers so Traefik is doing configured health checks.

https://doc.traefik.io/traefik/routing/services/#health-check

Configure health check to remove unhealthy servers from the load balancing rotation. Traefik will consider your servers healthy as long as they return status codes between 2XX and 3XX to the health check requests (carried out every interval).

[josegonzalez]

It's not super clear how healthchecks are configured with docker labels, but assuming that is sorted, we can probably just translate our existing app.json-based healthchecks to the traefik docker label format.