add env var PROXY for overridable "proxy" for react-scripts dev server

facebook#8598

Author: Viacheslav Lotsmanov

packages/react-scripts/scripts/start.js

@@ -59,6 +59,7 @@ if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
 // Tools like Cloud9 rely on this.
 const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
 const HOST = process.env.HOST || '0.0.0.0';
+const PROXY = process.env.PROXY || null;
 
 if (process.env.HOST) {
   console.log(
@@ -121,7 +122,7 @@ checkBrowsers(paths.appPath, isInteractive)
       webpack,
     });
     // Load proxy config
-    const proxySetting = require(paths.appPackageJson).proxy;
+    const proxySetting = PROXY || require(paths.appPackageJson).proxy;
     const proxyConfig = prepareProxy(
       proxySetting,
       paths.appPublic,