Nginx as forward proxy and authentication

Question

I want to set up an environment where I would have a simple implication that will be accessible through a forward proxy using for both the proxy and the app nginx.

Both of my nginx are docker containers.

First this is the nginx.cong of the application

events { }

http {
  server {
    auth_basic "Restricted Content";
    auth_basic_user_file /etc/nginx/.htpasswd;
    root /doc;
    location / {

    }
  }
}

It is working fine with the user:password that I set up. I am able to access it doing a simple curl command

curl -u user:password http://172.18.0.1:8184/test.csv

Now, I set up the proxy using this configuration

events { }


http {

    server {
        auth_basic "BLABLABLA";
        auth_basic_user_file /etc/nginx/.htpasswd;
        listen 8185;
        location / {
            proxy_pass http://$http_host$uri$is_args$args;
            proxy_pass_request_headers on;
        }        
    }
}

I try to access the first app again using this proxy doing this command

curl -x 127.0.0.1:8185 -U user_proxy:password_proxy -u user:password http://172.18.0.1:8184/test.csv

But it return 401.

In the log I see that the proxy is reading user:password of the app and not the one provided as user_proxy:password_proxy.

I don't find how to set up the proxy to use correctly the right user:password as Authentication.

Thanks in advance.