PUID/GUID=0 means run as root according to PhotoStructure | PhotoStructure for Docker, which is not recommended.
I wrote Synology NAS Tips - #6 by nuk walking through on my Synology DS920+ with DSM 7.2 for creating a non-root user and getting it’s PUID+GUID for a Synology docker. I guess if @mrm wants to support running as root he’ll have to investigate this further, but I know other software explicitly won’t run as root so I could see him going either way.
Also see the next comment after that for how I set up my docker container in general, in case that’s helpful to you.