[vue build] with webpack.config.js, Component css lost in index.css

[strongdevx]

my component like this:

<template>
...
</template>
<script>
//some code here
</script>
<style>
 .content{
  width:500px;
  height:600px;
}
</style>

I want to use vue build create umd bundle.

when I use vue build,the result file index.css include the style content. like this:

vue build my-component.vue --prod --lib

But when I use vue build with webpack.config.js，the style(css) lost. like this:

vue build my-component.vue --prod --lib --webpack webpack.config.js

My webpack.config.js：

var path = require('path');
var utils = require('./utils');
var webpack = require('webpack');
var merge = require('webpack-merge');
var webpackBaseConfig = require('./webpack.base.conf.js');
var ExtractTextPlugin = require('extract-text-webpack-plugin');

process.env.NODE_ENV = 'production';

module.exports = merge(webpackBaseConfig, {
  entry: './index.js',
  output: {
    path: path.resolve(__dirname, '../dist'),
    publicPath: '/dist/',
    filename: 'index.js',
    library: 'leaflet-map',
    libraryTarget: 'umd',
    umdNamedDefine: true
  },
  externals: {
    vue: {
      root: 'Vue',
      commonjs: 'vue',
      commonjs2: 'vue',
      amd: 'vue'
    }
  },
  plugins: [
    // @todo
    new webpack.DefinePlugin({
      'process.env.NODE_ENV': '"production"'
    }),
    new ExtractTextPlugin({
      filename: utils.assetsPath('css/[name].[contenthash].css')
    })
  ]
});

Is something missing from me?