Yes, Wordpress/PHP stores data in the /var/www/html directory.
I modified my copy of the docker-compose.yml file to add a volume and mount for that directory so it persists.
docker-compose.yml
version: '3.4'
services:
db:
image: mysql:5.7
volumes:
- db_data:/var/lib/mysql
restart: always
environment:
MYSQL_ROOT_PASSWORD: somewordpress
MYSQL_DATABASE: wordpress
MYSQL_USER: wordpress
MYSQL_PASSWORD: wordpress
wordpress:
depends_on:
- db
image: gforghetti/wordpress:latest
volumes:
- php_data:/var/www/html
ports:
- "8000:80"
restart: always
environment:
WORDPRESS_DB_HOST: db:3306
WORDPRESS_DB_USER: wordpress
WORDPRESS_DB_PASSWORD: wordpress
WORDPRESS_DB_NAME: wordpress
volumes:
db_data: {}
php_data: {}