https://stackoverflow.com/questions/37015624/how-to-run-a-cron-job-inside-a-docker-container https://crontab.guru/#*/1__**
Actually it should be done same way as written in official documentation. Data volume container stores it's data in "virtual root", so you should backup with next command:
docker run --rm \
--volume [DOCKER_COMPOSE_PREFIX]_[VOLUME_NAME]:/[TEMPORARY_DIRECTORY_TO_STORE_VOLUME_DATA] \