How to build a project with dev config in vue.js webpack template?

Question

webpack

I use Vue.js and its webpack template ( https://github.com/vuejs-templates/webpack)
The template provides npm run dev and npm run build commands. The first runs a local server for development, the second builds for production.
This template has /config/dev.env.js and /config/prod.env.js files, where you can stuff various variables, in particular API_URL.
How can I build in the easiest way (not start a local server, but build) an application using variables from dev.env.js?
As I understand it, webpack.prod.conf is used for building, and it needs this line to be taken depending on the npm command, but if done in package.json
const env = require('../config/prod.env')
"build": "NODE_ENV=dev node build/build.js"
and then if-ami look at process.env.NODE_ENV and include different configs, then it didn’t work for me