title: Postcss-loader not minifying css output
body: <p>I'm using webpack and postcss-loader to autoprefix and minify my CSS, before loading it into css-loader to use css-modules. I'm having trouble with minifying CSS. Examining the emitted .js file by webpack shows the CSS isn't minified with cssnano (because I can still see whitespace characters). What am I doing wrong?</p>
  
  <p>Some relevant configuration files:</p>
  
  <p><strong>webpack.config.js:</strong> </p>
  
  <pre><code>const path = require('path');
  const webpack = require('webpack');
  
  var postCompileScript = require('./postCompile');
  
  module.exports = {
    entry: './src/popup.js',
    module: {
      rules: [
        {
          test: /\.js$/,
          exclude: /node_modules/,
          use: {
            loader: "babel-loader"
          }
        },
        {
          test: /\.css$/,
          use: [
            'style-loader',
            { 
              loader: 'css-loader', 
              options: { 
                modules: true,
                localIdentName: '[local]__[hash:base64:6]',
                importLoaders: 1,
                minimize: true
              }
            },
            {
              loader: 'postcss-loader',
              options: {
                ident: 'postcss',
                plugins: [
                  require('autoprefixer')({})
                ],
                minimize: true
              }
            }
          ]
        },
        {
          test: /\.(png|jpg|gif)$/i,
          use: [
            {
              loader: 'url-loader',
              options: {
                limit: 8192
              }
            }
          ]
        }
      ]
    },
    output: {
      filename: 'main.js',
      path: path.resolve(__dirname, 'dist')
    },
    mode: 'production',
    resolve: {
      alias: {
        "react": "preact-compat",
        "react-dom": "preact-compat"
      },
      extensions: ['.js', '.jsx']
    }
  };
  </code></pre>
  
  <p><strong>postcss.config.js</strong>:</p>
  
  <pre><code>module.exports = {
    parser: 'sugarss',
    plugins: {
      'postcss-import': {},
      'postcss-preset-env': {},
      'cssnano': {}
    }
  }
  </code></pre>
  
  <p><strong>package.json</strong></p>
  
  <pre><code>{
    "name": "REDACTED",
    "version": "1.0.0",
    "description": "",
    "private": true,
    "scripts": {
      "test": "echo \"Error: no test specified\" &amp;&amp; exit 1",
      "build": "webpack --config ./webpack.config.js",
      "deploy": "node ftp"
    },
    "author": "",
    "license": "ISC",
    "devDependencies": {
      "autoprefixer": "^9.1.2",
      "babel-core": "^6.26.3",
      "babel-loader": "^7.1.5",
      "babel-preset-env": "^1.7.0",
      "babel-preset-react": "^6.24.1",
      "css-loader": "^1.0.0",
      "cssnano": "^4.1.0",
      "ftp": "^0.3.10",
      "post-compile-webpack-plugin": "^0.1.2",
      "postcss-loader": "^3.0.0",
      "prepend-file": "^1.3.1",
      "style-loader": "^0.22.1",
      "uglifyjs-webpack-plugin": "^1.3.0",
      "url-loader": "^1.1.1",
      "webpack": "^4.16.5",
      "webpack-bundle-analyzer": "^2.13.1",
      "webpack-cli": "^3.1.0"
    },
    "dependencies": {
      "iframe-resizer": "^3.6.1",
      "js-cookie": "^2.2.0",
      "npm": "^6.4.0",
      "preact": "^8.3.1",
      "preact-compat": "^3.18.3",
      "react": "^16.4.2",
      "react-dom": "^16.4.2",
      "react-iframe-resizer-super": "^0.2.0"
    }
  }
  </code></pre>
  
  <p>Thanks in advance.</p>