This is a Discourse forum? And you’ve somehow installed it behind cPanel?
Is the docker container running? What do you get from
docker ps
and
docker inspect app
If this is a standard install, then someone changed the DNS for your site. Another thing that I’ve had happen with things like cPanel, is I tried to disable cPanel and did a standard install, but then every now and then some process would run and restart the cPanel-like thing, so I’d have to figure out how to stop Apache and restart the docker container.