I used this tutorial to integrate bootstrap in my project:
https://laravel-news.com/2015/10/setup-bootstrap-sass-with-laravel-elixir/
This places an app.css
You should in your app.scss before including bootstrap file set variable $icon-font-path value correctly, probably in your case it should be:
app.scss
$icon-font-path
$icon-font-path: '/css/fonts/';