How to give "extra" options to Docker containers?

I already asked this in another thread but then the discussion drifted away. So I think I should start a new thread:

I installed Watchtower like described in this video:
https://forum.openmediavault.o…?postID=177009#post177009

It works but per default it leaves the old Images in list. https://hub.docker.com/r/v2tec/watchtower/ mentions some options, for example --cleanup (for removing old images after updating). I tried to put it into "Extra Arguments" field right in front or behind of -v /var/run/docker.sock:/var/run/docker.sock but I get an error message when I try so save the container. Where do I have to specify those options? (for watchtower and in general)
In my understanding the "Extra Arguments" field should be the right place but as I wrote it didn't work for me.

Ok, thanks for this explanation. Nice to know that this problem is (theoretically) solved. Can you give a modest estimate when this feature will come to production repo? Meanwhile I'd like to try the cli solution.

I assume I have to be logged in as "docker-user"?
Is there a simple way to see the docker command that is generated and executed from the Docker plugin?
If not:
- where can I find it?
- this would be a nice plugin feature too.