Minor UI suggestion - add some explanatory text on how to set environment variables

I suspect we may need to add a message outlining that environment variables should be set in the web server config in production; the putenv() example is only intended for local testing.

Yes, I think the on-screen documentation should describe how to handle the case for production, not dev.

Environmental variables aren't a concept unique to key, and there are lots of ways to set them. It might be good to add a page to the docs giving an example of how to do this, or add a tutorial about how to debug this (using putenv() in settings.php as suggested here, or, since a lot of people are using ddev now, .ddev/.env).