Justoverclock okay. So we found the main problem 🙂
Once you write the file (Dockerfile) to configure the containers, you should check the logs to make sure you don't get any errors and copy the basic dependencies of the services. In this case, you are trying to mount an Nginx server without its main configuration file.
Copy this one to your root folder to continue with the process:
https://github.com/zgq354/flarum-docker-env/blob/master/nginx/conf/nginx.conf
It's an example that will eliminate your problem, but you'll probably need to modify the content as you prefer.
Since I assume you want to continue learning, run this command to build the containers and you will find the following challenge on your way:
docker-compose build --no-cache
Spoiler: