env value loading

[steveoh]

Environment values that are set with an npm script like env-cmd -f ./.env.stage npm run build are overwritten if a .env.production file is present with different values.

possibly related to #9037

I am unsure how to use .env.development, .env.stage, and .env.production files effectively.

This is problematic for a few reasons but mainly since development and prod environments run on the root but staging runs on a /folder. Setting REACT_APP_BASENAME becomes hard to do.