Thanks both, that makes a lot of sense!
Two questions: how would I set up that ssd? I mean would it be part of the array as any other data disc, and then maybe I just choose to put the docker in there? From the Unraid docs I understand there are no disks left roaming free in the machine, meaning every unit is somehow part of the array, whether for parity, data, cache and so on.
Second Q: what is meant exactly by cache drive in Unraid? Would it be an additional drive that I'd add to the ssd dedicated to dockers?