To set the app icon in MacOS dock, the app.dock.setIcon function.

const nativeImage = require('electron').nativeImage
if (app.dock) {
  const image = nativeImage.createFromPath('icon.png')
  app.dock.setIcon(image);
}

Documentation on the dock, https://www.electronjs.org/docs/latest/api/dock#docksetmenumenu-macos