sp-dev-fx-webparts/tutorials/tutorial-getting-started/documentcardexample-webpart/gulpfile.js

'use strict';

const gulp = require('gulp');
const build = require('@microsoft/sp-build-web');

build.configureWebpack.mergeConfig({  
    additionalConfiguration: (generatedConfiguration) => {
        if (build.getConfig().production) {
            var basePath = build.writeManifests.taskConfig.cdnBasePath;
            if (!basePath.endsWith('/')) {
                basePath += '/';
            }
            generatedConfiguration.output.publicPath = basePath;
        }
        else {
            generatedConfiguration.output.publicPath = "/dist/";
        }
        return generatedConfiguration;
    }
});

build.initialize(gulp);