host not found in upstream "app" in /etc/nginx/conf.d/default.conf:16(Docker)

i am trying to dockerize my project

when i run docker compose up it works fine and when i access my myphpadmin port(3400), it works fine but if i try to access my nginx port (8080) i get this error

This site can’t be reached
127.0.0.1 refused to connect.

same error with port(8081)

and when i run docker log nginx

i get this message

host not found in upstream "app" in /etc/nginx/conf.d/default.conf:16

i have tried different method and i still get this message below is the content of my docker-compose.yml file

version: '3.8'

services:

  nginx:
    container_name: nginx
    image: nginx:alpine
    ports:
      - "8080:80"
    depends_on:
      - app
      - db
    volumes:
      - ./src:/var/www
      - ./nginx/conf.d/:/etc/nginx/conf.d

  db:
    image: mysql
    container_name: mysql
    ports:
      - "3307:3307"

    volumes:
      - ./mysql/data:/var/lib/mysql
    environment:
      - MYSQL_DATABASE=productservice
      - MYSQL_ROOT_PASSWORD=password

  phpmyadmin:
    image: phpmyadmin/phpmyadmin
    container_name: phpmyadmin
    ports:
      - "3400:80"
    depends_on:
      - db

  app:
    image: debian
    container_name: debian
    ports:
      - "8081:80"

and this is the content of my default.conf file

server {
    listen 80;
    index index.php index.html index.html;
    error_log /var/log/nginx/error.log;
    access_log /var/log/nginx/access.log;
    server_name localhost;
    root /var/www/public;

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

    location /index.php {
        try_files $uri = 404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass app:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
    }
}

please what am i doing wrong, i am new to docker please help