[Feature] Handle user/group run by unison

[wadjeroudi]

It would be nice to add this feature like UNISON_OWNER
https://github.com/mickaelperrin/docker-unison-unox

[kevinsmith]

I would second this. I believe this would alleviate the problem I'm running into, which is that all synced files end up being owned by root in the root group. This causes major issues for the application, which expects to see all files owned by www-data.