Edge Side Includes (ESI) with Varnish work basically like a charm. But if you want to dynamically add new backends the things became much more complicated. Traefik as reverse proxy can help here.
Traefik is able to monitor docker services and allows to update the configuration automatically without a restart. In this example the Vernish Cache is configured as an entry point with only one backend, the Traefik reverse proxy. All docker services register themselves using traefik.frontend.rule labels dynamically. You can add new services without any additional configuration.
We'll use two options of Traefik, defining a path mapping for frontend a and adding a hostname for frontend b.
Let's start the reverse_proxy and vernish containers first:
docker-compose start varnish reverse-proxy
A HTTP GET request to http://localhost/a or http://b.localhost should lead to a 404 error and that's fine for now.
So let's bring the first front end service up
docker-compose start frontend-a
While http://b.localhost remains not available a GET request to http://localhost/a URL should return some content now. Some parts are still missing, so let's start the last service
docker-compose start frontend-b
Now both side should be available and complete.