I haven’t tried anything else with this recently, but to answer your question: I am not running Nextcloud in docker, nor do I have any plans to do so. I originally installed from snap and then found smbclient was not included in the snap. This was a deal breaker so I threw out the snap and installed from zip. It’s working fine on that end.
I have tried both Collabora and OnlyOffice following information found in a multitude of guides and tried a variety of configurations including with/without proxy and same/separate servers. There is zero possibility of a connection issue because I can do a packet capture and see them talking back and forth. None of it works.
I can see that Nextcloud is sending a bunch of commands to Collabora that Collabora doesn’t like (responds with HTTP 403). I have absolutely entered the Nextcloud server address correctly and have checked it many times and redone it many times. I would very much like to get it working, but I’m at a loss for what else I can do to troubleshoot it.