How do I use environment variables in nginx to choose conditional upstream config

Question

I have an environment variable that I would like to set while starting an nginx service. Let's say the environment variable is NGINX_SERVICE which can have value DEV or QA. And I want to conditionally set upstream config.

ie, Logically something like this.

If ENV['NGINX_SERVICE'] == "DEV" 

    upstream backend {
       server localhost:2222;
    }

If ENV['NGINX_SERVICE'] == "QA" 

    upstream backend {
       server qa-server1:2223;
       server qa-server2:2223;
    }

Any idea how to accomplish this ?