Failing to get deploy builds to work

I've configured stages in my yml and each project where I want them.
As far as I understand, once this is done a new builder (deploy) is added that can be triggered from the Deploy page.
The build fails for me as the deploy builder apparently fails to do very basic stuff like read the "matrix" entry (actual error is `NameError: name 'TESTS' is not defined`)
Likely I'm doing something wrong, any suggestions? Should I not use 'matrix'?
Also how can I have "deploy" builder specific stuff?
After much serching I can't find any examples anywhere. Any help is appreciated.

Start with the yml stages and project configuration described in the issue, then reproduce the deploy builder from the Deploy page using the matrix entry. Compare the generated deploy configuration with the reported NameError; the issue is resolved when the deploy build handles the matrix and deploy-specific settings are documented or work as expected.

Written by the indexing model from the issue text.