We have a nextcloud server running together with a reverse proxy (both under the same apache) and a collabora docker instance. All on the same server. Our nextcloud server requires client certificates. There does not seem to be a way to have collabora online use a client certificate when sending requests to the nextcloud server. Please add support for this or explain how it is to be done.
I managed to circumvent the problem by adding a dummy network interface and a virtual host listening to it's address and serving nextcloud without requiring a client certificate. Using docker's '--add-host=' collabora/code accesses the nextcloud server on the new dummy address. So now I have two virtual hosts for the same nextcloud instance. One for users from outside and one for collabora from the inside. Still I think having collabora using client certficates would be better.
Since you have reported this for Collabora Online, if you still have this problem, please open an issue at: https://github.com/CollaboraOnline/online/issues