MariaDB writes more the 30GB/day temp data

I´ve a Synology NAS where I´m running a Nextcloud in a Docker Container and a MariaDB in an other Docker Container. On a day the the NAS and also the Nextcloud doesn´t do anything, on my SSD are over 30GBs written. So I´ve looking for a while and saw with *iotp*, that every time when the Nextcloud cron job is running, the MariaDB writes with over 20MB/s to my SSD. I tried to prevent it by using an ramdisk like here written: https://dba.stackexchange.com/questions/30505/why-does-mysql-produce-so-many-temporary-myd-files/30635#30635 For this I created the MariaDB Docker Container with

docker container run -d \
--mount type=tmpfs,destination=/tmp,tmpfs-size=1g,tmpfs-mode=1777 \
....

but when the Nextcloud cron job is running, I still see that the Mariadb writes with over 20MB/s to my SSD :( In the Container it looks fine:

df -h
Filesystem         Size  Used Avail Use% Mounted on
tmpfs              1.0G   13M 1012M   2% /tmp

with a script I´ve found witch uses *inotifywait* I see hat during the Nextcloud cron job a “/tmp//#sql-temptable-1-575-0.MAI” is created. I´ve also looked with

touch test
=> run the cron job
find / -newer test | grep -v "^/sys/" | grep -v "^/proc/"

if other things where wirtten, but it doen´t seems so:

/dev/pts/3
/dev/pts/ptmx
find: ‘/proc/1470/task/1470/fd/6’: No such file or directory
find: ‘/proc/1470/task/1470/fdinfo/6’: No such file or directory
find: ‘/proc/1470/fd/5’: No such file or directory
find: ‘/proc/1470/fdinfo/5’: No such file or directory
/tmp
/var/lib/mysql/ib_logfile0

Does anyone has an idea why the ramdisk not working or I can see what is written by the MariaDB and prevent it?