Error when exporting

Follow those notes.

Instead of running mysql -u cms -p cms inside the web container, you instead run:

mysqldump -u cms -p cms > /var/www/cms/library/temp/backup.sql

That will export the database to your CMS library temp - available as shared/cms/library/temp outside the container.

The containers take an automatic daily backup and put that in shared/backup, as well as an automatic backup on upgrade.