なんでかはよくわからいけど、volumesでリソース?を定義してそれをマウントしようとするとうまくいかなかったので、直接書いたらいけた
Docker for WindowsをWSLで使ってる

db:
image: mariadb:latest
environment:
MYSQL_ROOT_PASSWORD: password123
networks:
mynet:
ipv4_address: 172.21.0.2
volumes:
- mysql-vol:/var/lib/mysql
volumes:
wp-vol:
driver_opts:
type: none
device: /mnt/c/container/Docker/wordpress/volumes/html
o: bind
mysql-vol:
driver_opts:
type: none
device: /mnt/c/container/Docker/wordpress/volumes/data
o: bind
以下のように変更
db:
image: mariadb:latest
environment:
MYSQL_ROOT_PASSWORD: password123
networks:
mynet:
ipv4_address: 172.21.0.2
volumes:
- /mnt/c/container/Docker/wordpress/volumes/data:/var/lib/mysql
# volumes:
# wp-vol:
# driver_opts:
# type: none
# device: /mnt/c/container/Docker/wordpress/volumes/html
# o: bind
# mysql-vol:
# driver_opts:
# type: none
# device: /mnt/c/container/Docker/wordpress/volumes/data
# o: bind