Job breaks without an error after some min

Hi together,
the job first starts running well and also generating some preview… but than stops without any error message in console.

I run all in a docker environment. (nextcloud_fpm) with increased memory-limits of 2 GB
I also configured nextcloud (config.php) wich file type should be included…

running the command with -vvv gives me the following:

docker exec nextcloud_fpm /bin/bash -c 'php -f /var/www/html/occ preview:generate-all bannyAdmin -vvv' 

2018-06-15T12:43:10+00:00 Scanning folder /bannyAdmin/files
2018-06-15T12:43:10+00:00 Scanning folder /bannyAdmin/files/idm.bilder
2018-06-15T12:43:10+00:00 Scanning folder /bannyAdmin/files/idm.bilder/Interne Bilder
2018-06-15T12:43:10+00:00 Scanning folder /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent
2018-06-15T12:43:10+00:00 Generating previews for /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent/DSC_0001.png
2018-06-15T12:43:10+00:00 Generating previews for /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent/DSC_0002.png
2018-06-15T12:43:10+00:00 Generating previews for /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent/DSC_0003.png
2018-06-15T12:43:10+00:00 Generating previews for /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent/DSC_0004.png
2018-06-15T12:44:18+00:00 Generating previews for /bannyAdmin/files/idm.bilder/Interne Bilder/2016 - Idm - Secundum - Nachevent/DSC_0005.png

and than the command returns. if I run it next time I get the same output but. sometimes only till DSC_0004.png, sometimes till 0005.png… any idea ?