What I meant is to go to the /var/discourse directory (it has the discourse_docker git repository in it). If you just create a directory named discourse_docker you will install a new discourse. What I meant is to run the following (and only the following):
cd /var/discourse
git pull
./launcher rebuild app
Just make sure to stop the new discourse you started.
(Or you can just start a new discourse and restore it)
I didnāt create this folder. It was already there 
New discourse forum is now setup (yes working with nginx) ^^.
Just had a problem with sending activation email but I will deal with that on another topic.
Thank you all for helping.
Where?
Good to know. Didnāt you want to use an already existing discourse site? But if that worked in your case then itās fine too 
I didnāt want to use any existing discourse. I wanted to do fresh installation for a brand new domain.