Just wanted to let you know that finally Collabora Online in Nextcloud 10 works as expected on my Debian Stretch system. The update of Collabora Online App (Richdocuments) from 1.1.7 to 1.1.9 fixed the last issue for me, see #4481: Failed to get LoKitDocument instance
By the way, I still use the Collabora Online Docker container and I don’t use aufs. In Debian Stretch (with Linux kernel 4.7), AFAIK Docker uses the overlay storage engine per default. And apparently that works well with the Collabora Online Docker container.
So the bottom line is that I run a Debian Stretch VM (Linux kernel 4.7) with Nextcloud 10, latest Collabora Online Docker container and Richdocuments/Collabora Online App 1.1.9. The Apache VHost is configured according to the offical docs: Collabora Online in Nextcloud. I have HTTP->HTTPS redirection enabled and use LetsEncrypt SSL certificates for both the Nextcloud and the Collabora VHosts.