Health Ready

Readiness probe for the Docker HEALTHCHECK (Dockerfile). Traefik's docker provider drops a container from the load balancer as soon as it turns unhealthy, so answering 503 from the moment SIGTERM arrives pulls this instance out of rotation while it can still serve, instead of after its socket is gone. Only SIGTERM flips it: a drain triggered by the instance marker starts while the new container is still booting, and going unready then would leave nobody routable.