example for extend ionic/app-scripts/config/webpack.config.js

webpack.config.js

// don't forget to install "lodash" "dotenv" 

var ionicWebpackConfig = require('@ionic/app-scripts/config/webpack.config.js');
var webpack = require('webpack');
var _ = require('lodash');
var dotenvConfig = require('dotenv').config();
var dev = ionicWebpackConfig.dev;
var prod = ionicWebpackConfig.prod;

var loadProcessEnvValues = function() {
  return new webpack.DefinePlugin({
    'process.env': _(process.env)
                    .pick(_.keys(dotenvConfig.parsed))
                    .mapValues((v) => (JSON.stringify(v)))
                    .value()
  });
};

dev.plugins.push(loadProcessEnvValues());
prod.plugins.push(loadProcessEnvValues());

module.exports = {
  dev: dev,
  prod: prod
}

Cool, thank you!