How to ignore APP_URL for getFirstMedia() method for responsive images

[stefanofinetti]

Basically, I have a laravel installation which covers multiple domains (with a middleware), thus rendering completely useless the APP_URL variable. More, the variable should not be used because url will never have what is in APP_URL value. I've already done some override to the UrlGenerator, but it seems I can't get past use of APP_URL value for getFirstMedia() method, which I'm using to generate responsive . I can't find where APP_URL is read so I can't do another override unless you point me to where I should look. Thanks