How do I get PHP-FPM to work with nginx-proxy with FastCGI?

Daniel's answer is definitely on the right track. I use the php-fpm image with nginx as my main stack for php sites. Having said that, I don't use the nginx-proxy docker image. Instead, I use plain nginx on the host machine, and configure ports to point to backend php-fpm docker images.

I'm not using docker-compose either. Since it's just docker containers running single sites, I don't need it. Here's an example docker run command:

docker rm -f www.example.com || true
docker run -itd -p 9001:9000 -P \
        --name www.example.com \
        --volume /var/www/html/www.example.com:/var/www/html/www.example.com \
        --link mariadb:database.example.com \
        --restart="always" \
        --hostname="example.com" \
    --log-opt max-size=2m \
    --log-opt max-file=5 \
        mck7/php-fpm:7.4.x-wordpress

And here is an example nginx config:

server {
  server_name example.com www.example.com;

  location ~ /.well-known {
    allow all;
  }

  location ~ /\.ht {
    deny all;
  }

  root /var/www/html/www.example.com/src;

  index index.php;

  location / {
    try_files $uri $uri/ /index.php?$args;
  }

  location ~ [^/]\.php(/|$) {
    fastcgi_split_path_info ^(.+?\.php)(/.*)$;
    if (!-f $document_root$fastcgi_script_name) {
      return 404;
    }

    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_param PATH_INFO       $fastcgi_path_info;
    fastcgi_param PATH_TRANSLATED $document_root$fastcgi_path_info;

    fastcgi_pass   127.0.0.1:9001;
    fastcgi_index  index.php;
  }
}

A few things about this setup are key. The port re-mapping for the docker container. In this example I map port 9001 to 9000. The other "gotcha" is that the root for the container must be an actual location on the host. I have no idea why this is the case, but for whatever reason the path docker thinks it's using has to actually be the path on the host as well.

The default generated config of nginx-proxy is not fully working.

I think something is messed up with VIRTUAL_ROOT environment variable, because the root of the problem is PHP getting a wrong path via SCRIPT_FILENAME (that's why you see no PHP output) and there is no try_files with =404 symbol (that's why you get 200 with everything).

I have a prepared working setup using docker-compose in GitHub to demonstrate that it would work with an existing SCRIPT_FILENAME in the nginx config.

I have changed test.local to test.localhost.

I think to get it working as it should, you would have to use an nginx template for nginx-proxy, so the generated default.conf does work with php fpm and have the missing fastcgi param included.

Another, yet different approach would be to pack PHP and a manually configured webserver (nginx) in a project and having the automated reverse nginx proxy in a standalone project. This would cost you an additional process running but gives you more control and easier deployment.

Alternatively, you might want to have a look into traefik which does essentially the same as nginx-proxy.