In what context do Cypress Tests run when using Github Integration?

[mattbloomfield]

I'm using Platform.sh for my host.
The application is CraftCMS and is has a fairly lengthy build process (4 min).
Once a new PR is opened in Github, it sends a webhook to Platform.sh which builds a new container and returns a dedicated URL for that environment.

I've enabled the Cypress Github Integration via the Dashboard. Theoretically, I want to run my tests every time a PR is opened, but after the environment is setup. This is my setup in Cypress:

But how do I tell Cypress what the Base URL is for the tests? I don't know it until the environment is built. Do the cypress tests run in my Platform.sh container? Or are you running them in your own environment? I'm just confused about that part.

Running locally everything makes sense, just struggling with the integration aspect.