I’m not that experienced with Docker (in production) yet, but have met a lot of the pain points.
In the ideal world, Docker would be running on a minimal OS eg. Debian by itself. No need for Symbiosis. It doesn’t matter (to some extent) which version of Debian you were running as Docker isolates your container from the specifics of the under lying OS. Hence the ability to build & test in one environment, and deploy it elsewhere in the knowledge that it will mostly work just fine.
Docker could sit alongside of your current Symbios, but see my comments about firewalls.
It looks like you want a mixture of your current Symbiosis and Nextcloud.
I don’t know nextcloud, but if it needs Debian 9 and you are running a lower version under Symbiosis, you might need to upgrade and all the pain involved wit that.
If you use Docker you can run it on an earlier version of Debian, as the Docker image will come with its own mini-version of Debian 9 inside.
I’m rambling …