Just a few comments on the ability to use a folder / share for docker
If you're one of those users who continually has a problem with the docker image filling up, this is the solution, as the "image" will be able to expand (and shrink) to the size of the cache drive. Just be aware though that this new feature is technically experimental. (I have however been running this on an XFS formatted cache drive for a while now, and don't see any problems at all)
I would recommend that you use a share that is dedicated to the docker files, and not a folder from another existing share (like system as show in the OP).
My reasoning for this is that
If you ever have a need to run the New Permissions tool against the share that you've placed the docker folder into, then that tool will cause the entire docker system to not run. The folder will have to be removed (via the command line), and then recreated.
All of the folders contained within the docker folder are not compatible with being exported over SMB, and you cannot gain access to them that way. Using a separate share will also allow you to not export it without impacting the other shares' exporting. (And there are no "user-modifiable" files in there anyways. If you do need to modify a file within that folder, (ie: a config file for a container and that config isn't available within appdata), you should be doing it via going to the container's shell)
You definitely want the share to be cache only (although cache prefer should probably be ok). Setting it to cache:yes will undoubtedly cause you problems if mover winds up relocating files to the array for you.
On this beta (until the GUI properly supports this new feature), you also cannot use Settings - Docker to stop / start the service if you've made the change to the .cfg file to utilize this feature. (You can stop the service, but in order to restart it you have to enable it via the config file and then stop / start the array)
I did have some "weirdness" with using a Unassigned Device as the drive for the docker folder. This may however been a glitch in my system.
Fix Common Problems (and the Docker Safe New Permissions Tool) will wind up getting updated (once the GUI properly supports these changes) to let you know of any problems that it detects with how you've configured the folder.