Check if configuration file is valid
docker exec -it <CONTAINER_ID> haproxy -c -f /config/haproxy.cfg
HAProxy Config
The configuration file is stored on the VM under /opt/ha-proxy-certbot/config/haproxy.cfg
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