Add healthcheck before deploy

[dhh]

Ensure your app responds with 200 OK to GET /up before allowing a deploy or redeploy to go through.

MRSK defaults to checking the health of your application again /up on port 3000. You can tailor both with the healthcheck setting:

healthcheck:
  path: /healthz
  port: 4000

This will ensure your application is configured with a traefik label for the healthcheck against /healthz and that the pre-deploy healthcheck that MRSK performs is done against the same path on port 4000.