Is it possible to ./launcher rebuild app without updating discourse?
You could, but you would have to specify the exact commit hash in your app.yml file.
Can you please give me an example how this would be setup?
and replace tests-passed with the commit hash something like
You can find the commit hash in github if you click on the commit that you want.
How can find necessary commit ?
For example, I have version 2.4.0 beta2 and want to stay on this version.
Go to github and look at tags and you can choose the one for the version you want.