Adding reverse proxy setup to Nginx configuration of Matomo container 'breaks' the website?

Question

thanks for any help.

I am setting up Matomo 4.14.2-fpm (docker container). I have successfully configured it with MariaDB, and I can access it at https://localhost/ (after clicking through the no-cert warning since I haven't set up the cert yet), with nginx configured for SSL.

However, when I add the proxy_pass settings (described below), when I access http://matomo/ (also http://matomo:80), it shows the NGINX welcome message (there's an unconfigured nginx container running on http://matomo/). https://localhost/ shows the same thing. Nginx container logs look normal, as do Matomo container logs.

I followed the instructions here: https://matomo.org/faq/how-to-install/faq_98/, so the nginx conf has:

location / {
    proxy_pass http://matomo:80/;
    proxy_set_header X-Forwarded-Host $host;
    proxy_set_header X-Forwarded-Server $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

    client_max_body_size 250M;
}

and config.ini.php has:

[General]
force_ssl = 1
assume_secure_protocol = 1

proxy_client_headers[] = "HTTP_X_FORWARDED_FOR"
proxy_host_headers[] = "HTTP_X_FORWARDED_HOST"

I have looked at other people asking for help on similar topics but it mostly seems to be advising them to add the proxy headers to config ini php, which I've already done. Admittedly, most of my config is cobbled together from multiple sources and I don't understand reverse proxy too well, so I think it could just be me not understanding host reverse proxy works. Any help is much appreciated.