Adding postcss plugins to Gridsome

gridsome.config.js

module.exports = {
  siteName: 'Example',
  chainWebpack: config => {
    config.module
      .rule('postcss')
      .oneOf('normal')
      .use('postcss-loader')
      .tap(options => {
        options.plugins.unshift(...[
          require('rucksack-css'),
        ])
        return options
      })
  },
}