As we already know, we can use all the branches of the discourse repo in our app.yml file. Similarly is that we can also use release tags. These are the release tags of the discourse repo.
https://github.com/discourse/discourse/tags
And the good news is, nothing extra is required. If you want to use v2.5.0 just put version: v2.5.0 in your app.yml file.
~~Note: You cannot use commit hashes here. Read https://stackoverflow.com/questions/14370157/git-fetch-a-specific-commit-by-hash~~
You can actually use commit hashes too.