The versions we are running are half a year old. I tried to upgrade them, assuming that the Docker images have a working updater integrated, but apparently not. So we have to find some way to upgrade our OnlyOffice images to the latest version.
Unfortunately the documentation is not helpful. Upgrade instructions are essentially "run the new docker image with the old data" (which is what I did). Maybe we have to install it again from scratch and then import the old data.