You specify the path of the docker image when you enable docker service in Settings - Docker. The default is in the system share. There is absolutely no reason to back it up. In fact, we often recommend people delete it when they have some kinds of problems, so it can be recreated.
The docker image contains the executable code of all your containers, and these are easily downloaded again. The Previous Apps feature on the Apps page will reinstall your dockers just as they were.
Possibly what you are thinking of when you say "config" is the mappings and other things on the Add/Edit Container page. These are saved in a template on the flash drive when you create/edit a container, and these templates can be reused by simply selecting the template on the Add Container page, or more simply, by using the Previous Apps feature previously mentioned.
So, a current backup of flash is also important. It is especially important to have a flash backup with the current disk assignments. You can download a zipped backup of flash at any time by going to Main - Boot Device - Flash - Flash Backup. Or you can schedule an automatic backup of flash with CA Backup.
It seems like all the reasons you are considering removing cache has better solutions. And letting Unraid manage cache as part of user shares is much simpler.