Add a test site to a LAMP+ElasticSearch docker compose cluster frontend by Traefik

We have a Docker Swarm fronting an ElasticSearch cluster and a Drupal instance. The config is at docker-compose.yml · GitHub

I am just the Drupal backend developer with a few years (decades...) of Linux, I just don't understand how these pieces fit together. I do not even know whether this is correct to distribute the load between the ElasticSearch nodes / make it highly available. I do know we wanted to be the drupal URL but somehow ended up with -- I had someone configure traefik, it wasn't me, but they are no longer available.

Anyways, we would like to add a test site either on a different subdomain or a different port. A single node ElasticSearch would be enough. How would I do that? Thanks so much!