Nginx reverse proxy automatic redirects go to wrong port

I am trying to set up a reverse proxy to a docker container using nginx.

The docker container is available and working on port 8000.

I would like to be able to reach the container through the address mydomain.com:80/mycontainer.

The reverse proxy seems to succeed in talking to the container but when the container requests a redirect to its login page nginx tries to load the login page on port 80 instead of 8000 which fails.

this is my nginx config so far:

    upstream docker-container {
        server 127.0.0.1:8000;
    }

    server {
        listen 80;

        location /mycontainer {
            proxy_redirect     off;
            proxy_set_header   Host $http_host;
            proxy_set_header   X-Real-IP $remote_addr;
            proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header   X-Forwarded-Host $server_name;
            proxy_set_header   Upgrade $http_upgrade;
            proxy_set_header   Connection "upgrade";
            proxy_pass         http://docker-container;

        }

    }

This is what I get when I run wget -S 127.0.0.1/mycontainer:

wget -S 127.0.0.1/mycontainer
--2021-08-29 20:30:12--  http://127.0.0.1/mycontainer
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 302 Found
  Server: nginx/1.18.0 (Ubuntu)
  Date: Sun, 29 Aug 2021 20:30:12 GMT
  Content-Type: text/plain; charset=utf-8
  Content-Length: 28
  Connection: keep-alive
  Content-Language: en
  X-Frame-Options: SAMEORIGIN
  X-Download-Options: noopen
  X-Content-Type-Options: nosniff
  Referrer-Policy: origin-when-cross-origin
  X-XSS-Protection: 1; mode=block
  Location: /login
  Vary: Accept
  Set-Cookie: some cookie
Location: /login [following]
--2021-08-29 20:30:12--  http://127.0.0.1/login   <--- HERE IS THE PROBLEM. Should be 127.0.0.1:8000/login
Reusing existing connection to 127.0.0.1:80.
HTTP request sent, awaiting response...
  HTTP/1.1 404 Not Found
  Server: nginx/1.18.0 (Ubuntu)
  Date: Sun, 29 Aug 2021 20:30:12 GMT
  Content-Type: text/html
  Content-Length: 162
  Connection: keep-alive
2021-08-29 20:30:12 ERROR 404: Not Found.

I am by no means a professionnal at this so I am sure there are a lot of mistakes.

How can I get the automatic redirects to point to the correct port (8000) and not to port 80?