Currently, the option that exist is:
ctlptl docker-desktop set vm.fileSharing /Users,/Volumes,/private,/tmp
In order to use this, you need to know what's there already, not to overwrite it. If the list is long, it's cumbersome to copy-paste all the values, to add a new value.
It would be nice with a command like
ctlptl docker-desktop append vm.fileSharing /tmp
which would (if not already in the list) append /tmp to the list.