wp-theme:
image: toancong/wp-sage:sage9
volumes:
- ./wp-content/themes:/usr/source
docker-compose run --rm wp-theme bash -c "composer create-project roots/sage your-theme-name 8.5.1 && cd your-theme-name && npm install && bower install --allow-root"
docker-compose run --rm wp-theme bash -c "composer create-project roots/sage your-theme-name dev-master && cd your-theme-name && yarn"
See the theme in your backend /wp-admin/themes.php