Discourse data is written to the host, so whatever has been written, will survive.
Unless you’re on a very busy Discourse instance, I would say you could simply shutdown now.
On a very busy Discourse instance, I’d stop Nginx and DB (in Docker) manually, then shutdown the app and OS. I’d probably make a script that runs these steps and you can do the same (sudo ./shtudown-docker.sh isn’t that hard).