ERROR: Named volume "data:/var/lib/mysql:rw" is used in service "xxxx" but no declaration was found in the volumes section.
トップレベルにもvolumes
を指定しないとだめ。
version: "3" services: db: image: db volumes: - data-volume:/var/lib/db backup: image: backup-service volumes: - data-volume:/var/lib/backup/data volumes: data-volume: