nginx - nginx's reverse proxy returns index.html for static resources if proxy_pass contains variable

Question

I have the following nginx config (nginx is hosted in docker as well as a proxied resource):

location /coverage-22.1/ {
  resolver 127.0.0.11 valid=30s ipv6=off;
  set $url coverage-report-22.1;
  proxy_pass  http://$url/;
}

I use the $url variable to prevent the error 'host not found in upstream' as its suggested here: docker-nginx-stopped-emerg-11-host-not-found-in-upstream. The coverage-report-22.1 is a name of container in my docker.

This configuration helps to prevent the host not found in upstream error, but all requested resources (like http://myserver/coverage-22.1/js/site.js) return content of the index page.

The default configuration works perfectly but nginx fails with the mentioned error if a proxied url is not available:

location /coverage-22.1/ {
  proxy_pass  http://coverage-report-22.1/;
}