Yeah, right now your usage is ~20 odd Gig, excluding icons, various support files and what not.
There's nothing extravagant in a container storing excess data into the image. Based on the number of containers, the log for cabackup is also probably huge (but I'd be hard pressed to see it more than a gig or so) which is also stored within the image (simply because there's no other real place to store it)
I would just delete and recreate the image then monitor the situation. https://forums.unraid.net/topic/57181-docker-faq/#comment-564309