Add warning when HOST environment variable is set (facebook#3730)

* Add warning when HOST environment variable is set (facebook#3719)

* Improve HOST environment variable warning message

* Adjust text and message

Closes facebook#3719

scripts/start.js

@@ -51,6 +51,21 @@ if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
 const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
 const HOST = process.env.HOST || '0.0.0.0';
 
+if (process.env.HOST) {
+  console.log(
+    chalk.cyan(
+      `Attempting to bind to HOST environment variable: ${chalk.yellow(
+        chalk.bold(process.env.HOST)
+      )}`
+    )
+  );
+  console.log(
+    `If this was unintentional, check that you haven't mistakenly set it in your shell.`
+  );
+  console.log(`Learn more here: ${chalk.yellow('http://bit.ly/2mwWSwH')}`);
+  console.log();
+}
+
 // We attempt to use the default port but if it is busy, we offer the user to
 // run on a different port. `choosePort()` Promise resolves to the next free port.
 choosePort(HOST, DEFAULT_PORT)