Here is a solution for Docker in a script /bin/sh :
docker exec [MYSQL_CONTAINER_NAME] sh -c 'exec echo "[client]" > /root/mysql-credentials.cnf'
docker exec [MYSQL_CONTAINER_NAME] sh -c 'exec echo "user=root" >> /root/mysql-credentials.cnf'
docker exec [MYSQL_CONTAINER_NAME] sh -c 'exec echo "password=$MYSQL_ROOT_PASSWORD" >> /root/mysql-credentials.cnf'
docker exec [MYSQL_CONTAINER_NAME] sh -c 'exec mysqldump --defaults-extra-file=/root/mysql-credentials.cnf --all-databases'
Replace [MYSQL_CONTAINER_NAME] and be sure that the environment variable MYSQL_ROOT_PASSWORD is set in your container.
Hope it will help you like it could help me !