[Vuejs]-Merge WebPack config in Vue CLI Service 3

I found out the problem.

Project structure:

- src
  - main.js
  - ...
- package.json
- vue.config.js

I was launching build command from inside src directory, so vue.config.js was not considered.

However the correct solution for me was:

const DEV_MODE = process.env.NODE_ENV !== "production";
module.exports = {
  configureWebpack: {
     output: {
        publicPath: DEV_MODE ? '' : '/ui/'

     }
   }
}