HAProxy Config
The configuration file is stored on the VM under /opt/ha-proxy-certbot/config/haproxy.cfg and you need to be root to modify it. You can simply open the file, modify and save it.
There are four essential sections to an HAProxy configuration file. They are global, defaults, frontend, and backend. These four sections define how the server as a whole performs, what your default settings are, and how client requests are received and routed to your backend servers.
Check here: https://www.haproxy.com/blog/the-four-essential-sections-of-an-haproxy-configuration#what-about-listen
Check if configuration file is valid
docker exec -it <CONTAINER_ID> haproxy -c -f /config/haproxy.cfg