After I updated my Docker container on my Synology to v5.3.1, I noticed that it would not start up and kept rebooting.
The log and console showed the error message _ [✗] DNS service is not running_ and the container was automatically restarted.
After some research, I learned that the error can be resolved by providing a DNS server with the starting command, which is not possible to my knowledge on the Synology.
So the way I resolved the issue was to create an additional text file with the following content in the Pi-hole folder on the Synology:
nameserver 127.0.0.1 nameserver 126.96.36.199 nameserver 188.8.131.52
I mounted this file to the Pi-hole container at /etc/resolv.conf and the issue was solved.
Hope this helps if anyone stumbles over the same issue.