docker docker-compose nginx root directory in another container

I am having issues with my docker-compose to point the nginx container to serve the directory ${APP_CONTAINER_DIR} from my app container

right now, connecting to 127.0.0.1:8000 I get nginx error 404; I am guessing it is not finding the files since I have no error in the nginx logs

I think the problem comes from root /var/www/app/public; in dev.conf but not sure how to do it; I tried app:root /var/www/app/public; but this is not working

here is my docker-compose

version: "3"
services:

  nginx:
    container_name: nginx
    image: "${NGINX_IMAGE}"
    build: build/nginx
    restart: 'always'
    env_file: .env
    ports:
      - "8000:443"
    volumes:
      - "./build/nginx/build/dev.conf:/etc/nginx/conf.d/default.conf:ro"
      - "./build/nginx/build/certs:/etc/nginx/certs"
    networks:
      - app_network
    depends_on:
      - app

  app:
    container_name: app
    image: "${APP_IMAGE}"
    restart: always
    build: build/app
    env_file: .env
    volumes:
      - "${APP_HOST_DIR}:${APP_CONTAINER_DIR}"
    depends_on:
      - database
    networks:
      - app_network

  database:
    container_name: mariadb
    image: "mariadb:${MARIADB_VERSION}"
    restart: 'always'
    env_file: .env
    volumes:
      - "${SQL_INIT}:/docker-entrypoint-initdb.d"
      - "db_data:${MARIADB_DATA_DIR}"
      - "db_logs:${MARIADB_LOG_DIR}"
    environment:
      MYSQL_ROOT_PASSWORD: "${MYSQL_ROOT_PASSWORD}"
      MYSQL_DATABASE: "${MYSQL_DATABASE}"
      MYSQL_USER: "${MYSQL_USER}"
      MYSQL_PASSWORD: "${MYSQL_PASSWORD}"
    ports:
      - "3306:3306"
    networks:
      - app_network

volumes:
  db_data:
  db_logs:

networks:
  app_network:

dev.conf

#./images/nginx/build/default.conf

server {
    listen 443 ssl;
    server_name  127.0.0.1;
    ssl_certificate /etc/nginx/certs/dev.crt;
    ssl_certificate_key /etc/nginx/certs/dev.key;
    index index.php index.html;
    root /var/www/app/public;

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

    location ~ .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;
    }

    error_log /var/log/nginx/app.error.log;
    access_log /var/log/nginx/app.access.log;
}

.env

APP_HOST_DIR=./app
APP_CONTAINER_DIR=/var/www/app/