AWS ECS: Networking between container Nginx and PHP-FPM on the same host

So I have spent the past two weeks learning about ECS and trying to setup a very basic task with an nginx container and and a PHP app in another container. It seems like I'm very close, but I'm now getting this connection refused error from nginx: **[error] 29#29: *9 connect() failed (111: Connection refused) while connecting to upstream, client: 172.31.16.34, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://172.17.0.4:8000", host: "##########:48152"**

Here is my task definition from my CloudFormation template:

     ContainerDefinitions:
        - Name: nginx
          Cpu: 10
          Essential: true
          Image: ###################
          Memory: 128
          MountPoints:
            - ContainerPath: /var/www
              SourceVolume: my-vol
          PortMappings:
            - ContainerPort: 80
          Links:
            - app
        - Name: app
          Cpu: 10
          Essential: true
          Image: #############
          Memory: 128
          MountPoints:
            - ContainerPath: /var/www
              SourceVolume: my-vol
          PortMappings:
            - ContainerPort: 8000
      Volumes:
        - Name: my-vol
          DockerVolumeConfiguration:
            Scope: task
            Driver: local

My nginx Dockerfile:

FROM nginx:alpine
RUN apk update && apk add bash
COPY ./default.conf /etc/nginx/conf.d/default.conf

The config file:

server {
    listen 80;
    listen 443;
    index index.php index.html;
    error_log  /var/log/nginx/error.log;
    access_log /var/log/nginx/access.log;
    root /var/www/public;
    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass app:8000;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
    }
    location / {
        try_files $uri $uri/ /index.php?$query_string;
        gzip_static on;
    }
}

And my app Dockerfile:

FROM php:8.2-fpm
...
EXPOSE 8000

So, why am I using bridge mode, you ask? Well all of the examples I could find were using bridge mode, and I know that this is supposed to be like using networks in Docker, which I actually got working locally, so I thought this looked like the simplest solution. Also yes, I know that using Links is deprecated, but I couldn't find any recommended alternative.

So I can see that nginx is able to resolve the app host to the IP address of the container, so I'm guessing maybe the problem is on the PHP-FPM side, although in my app's logs I see fpm is running and ready to handle connections. Anyway I don't want to just go messing around making changes that I don't fully understand the consequences of. So if anyone could explain what's going on, that'd be great.