H Andreas,
I have not found within Synology NAS docker package any option to set the time zone.
The checkmk container is runnig with the wrong time:
root@checkmk-check-mk-raw2:/# date
Sun Jan 17 12:14:18 UTC 2021
while it is Sun Jan 17 13:14:18 CET 2021 on my NAS.
BUT … You made me look in to the correct direction 
In checkmk docker container I have added variable TZ with value Europe/Warsaw and voila:
root@checkmk-check-mk-raw2:/# date
Sun Jan 17 13:46:31 CET 2021
BUT in email notifications there is still UTC time given 
WTF ??!!