Nginx reverse proxy to subpage (non-root url)

Question

I want http://example.com to map to a subpage from somewhere else:

server {
    listen 80;
    server_name example.com;
    location / {
        proxy_pass http://somewhere.com/foo;
        proxy_redirect off;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-NginX-Proxy true;
    }
 }

The problem is that this page will load stuff like

<script src="/assets/script.js">

which will then be attempted to get loaded from http://example.com/assets/script.js - but this will actually go to http://somewhere.com/foo/assets/script.js instead of http://somewhere.com/assets/script.js

So how can I convince nginx to just map the root domain - http://example.com to http://somewhere.com/foo and everything else to http://somewhere.com/*?

I tried many things but couldn't get it to work...