Add a Docker healthcheck

Starting the full architecture using make in dev will cause the Ponos agent (ark-ponos) to start too early, before Django properly starts. The agent tries to load /api/v1/public-key, gets an HTTP 502 from Traefik, and cries. We could try to add a healthcheck, either in the backend Dockerfile or the architecture's Compose file. We can then use the long syntax of depends_on to set condition: service_healthy to cause Docker Compose to wait.