環境変数を使う方法 (env)
参考：
Environment Variables | Webpack [Official]
使い方
参考：
Passing Environment Variables Into Your Code With Webpack – Justin Tulk – Medium
Environment Variables | SurviveJS
Best way to pass variables through Webpack? · Issue #386 · gdi2290/angular-starter – GitHub
Using environment variables in React – Sam Pakvis – Medium
How can i pass webpack environment variables in html? – Stack Overflow
development と production を使い分ける
参考：
Implement NodeJS environment variables in a modern Webpack app | ITNEXT