Nginx proxy_pass to dynamic url captured with regex - nginx

Question

I'm trying to set a CORS proxy using Nginx.
My application runs on localhost:8080 (as a docker container) with Nginx. It calls some external resources thus facing CORS issues (remote servers do not have CORS enabled).
Here is the docker-compose file :
version: '3'
services:
myapp:
image: myimage
ports:
- 8080:80
volumes:
- ./apps:/usr/share/nginx/html/apps
- ./default.conf:/etc/nginx/conf.d/default.conf
And here is the default.conf file :
server {
listen 80;
server_name localhost;
#access_log /var/log/nginx/host.access.log main;
location ~ /proxy/(.*) {
proxy_pass https://$1$is_args$args;
access_log /var/log/proxy_access.log;
}
location / {
root /usr/share/nginx/html;
index index.html index.htm;
}
}
So my application calls http://localhost:8080/proxy/https://external_domain.com?key=value to bypass CORS restriction.
Problem is there seems to be an issue somewhere in my configuration because the previous request returns 500 error with the following message in logs :
invalid port in upstream "https://external_domain.com?key=value", client: <container_ip>, server: localhost, request: "GET /proxy/external_domain.com?key=value HTTP/1.1", host: "localhost:8080"
I tried alternative solutions using request url params instead of regex, but with no more success.