modalità debug su wordpress dockerizzato

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$ cat docker-compose.yml version: '3.3'
version: '3.3'

services:
   db:
     image: mysql:5.7
     volumes:
       - dbdata:/var/lib/mysql
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: somewordpress
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress

   wordpress:
     depends_on:
       - db
     image: wordpress:latest
     ports:
       - "8000:80"
     restart: always
     environment:
       WORDPRESS_DB_HOST: db:3306
       WORDPRESS_DB_USER: wordpress
       WORDPRESS_DB_PASSWORD: wordpress
       WP_DEBUG: 'true'
volumes:
    dbdata:

$ docker-compose up -d

$ firefox http://localhost:8000

Tutto sembra funzionare ma, facendo delle verifiche, la modalità debug non sembra sia stata attivata, nonostante abbia specificato nel file docker-compose.yml

WP_DEBUUG: 'true'

Perchè?