I was looking into setting up Nextcloud recently and the default directions suggest exposing the socket. That's crazy. I checked again just now. I see it is still possible to set it up without socket access, but that set of instructions isn't as prominent.
I linked to Docker in specific because if Nextcloud has access to the socket, and hackers find some automated exploit, they could easily escalate out of the Docker container. It sounds like you have it more correctly isolated.