feat: respect devServer field in webpack config as well

close #2053

Author: yyx990803

packages/@vue/cli-service/lib/commands/serve.js

@@ -42,12 +42,16 @@ module.exports = (api, options) => {
     const prepareProxy = require('../util/prepareProxy')
     const launchEditorMiddleware = require('launch-editor-middleware')
 
-    // load user devServer options
-    const projectDevServerOptions = options.devServer || {}
-
     // resolve webpack config
     const webpackConfig = api.resolveWebpackConfig()
 
+    // load user devServer options with higher priority than devServer
+    // in webpck config
+    const projectDevServerOptions = Object.assign(
+      webpackConfig.devServer || {},
+      options.devServer
+    )
+
     // expose advanced stats
     if (args.dashboard) {
       const DashboardPlugin = require('../webpack/DashboardPlugin')