Keep config-dependent environment vars in the config

NODE_ENV isn’t one we’d want to change dynamically, so keep it checked in to the config

.circleci/config.yml

@@ -7,6 +7,9 @@ jobs:
   build:
     docker:
       - image: circleci/node:8.16-browsers
+    
+    environment:
+      NODE_ENV: production
 
     working_directory: ~/repo