Hi @mevaha
It that is the case, then more-than-likely you have a reverse-proxy in front of your load balancer (or as a part of your load balancer).
Let me explain.
The reverse proxy (with load balancer if you have one) communicates with Discourse on the back end using HTTP.
So you are correct that Discourse communicates only HTTP, but only to the reverse proxy, not to the outside world.
So, it goes like this:
WEB USERS <-----> HTTPS <-----> REVERSE PROXY <----> HTTP <----> LOAD BALANCER <----> DISCOURSE (DOCKER)
Hence, you can expose your DISCOURSE Docker container as Port 80 (HTTP Only) as you are talking about.
However, on the web-facing site, you need a reverse proxy which is proxying the HTTPS requests to back end using HTTP.
Your reverse proxy (with load balancer), if set up correctly, will make sure HTTP cookies and headers are passed back and forth correctly.
Hope this helps.
If you have more questions, please ask.
Please note that if you kindly provide the exact technical details of your architecture it will be much easier to assist you; as we don’t have a crystal ball app working for us 