Set ENV variables in scripts instead of in build

.env.example

@@ -0,0 +1,13 @@
+##
+# EXAMPLE
+#
+# Read more about .env files when using create-react-app here:
+# https://facebook.github.io/create-react-app/docs/adding-custom-environment-variables#adding-development-environment-variables-in-env
+#
+
+PUBLIC_URL=/demo
+
+#
+# Most vars require REACT_APP_* naming
+#
+REACT_APP_CONFIG=config/netlify.js