This is a minimal setup based on a blog post written on blog.camunda.com.
docker-compose up
This will spawn one redis instance, 2 Camunda BPM containers, and nginx as load balancer bound to localhost using the session manager.
To test out how it works just visit http://localhost/camunda and login with the demo:demo credentials.
As we didn't define a database for Camunda BPM it is all kept in a local database per instance.
Now you can freely change up the name of the user and after refreshing a couple of times you will see that you are served by different instances but all sharing the same session.