docker mysql: mysqldump could not create dump file

docker mysql: mysqldump could not create dump file.

docker-compose.yaml

version: "3"services:  mysql:    container_name: mysql    image: mysql:8.0.33    environment:      - MYSQL_ROOT_PASSWORD=123456    user: 115:115    volumes:      - ./backup:/opt/backup

Tried:

sudo docker exec mysql mysqldump -u root -p123456  --no-tablespaces --databases my_db > /opt/backup/my_db.sql

Error:

-bash: /opt/backup/my_db.sql: No such file or directory 

The directory is there by:

sudo docker exec mysql ls -l /opt/backup

The owner of the directory is 115:115 (mysql). files can be created under the directory:

sudo docker exec mysql touch /opt/backup/foo.txt

If dump to current directory without directory "/opt/backup", no error message. But the dumped file could not be found under root directory. (current directory is root seen by "sudo docker exec mysql pwd").

Similarly, if dump to /tmp directory, no error message, but no file is created under the /tmp.

sudo docker exec mysql mysqldump -u root -p123456  --no-tablespaces --databases my_db > /tmp/my_db.sql

What is the way to dump mysql data from mysql docker container?