Well, I didn’t create it manually, of that I’m sure.
I did install Docker and Docker Compose via dietpi-software, and after setting up some apps in Portainer I noticed there were some directories like docker-data showing up in /mnt/dietpi-userdata, and then some of files that had been downloaded showed up in /DATA.
By then I’d already deleted all the containers I had installed via Portainer, so I wasn’t sure if those directories were a DietPi thing, or something else.
Sounds like it was probably just a mis-configured path issue from when I was messing with Portainer, so I’ll just get rid of those folders and try to pay more attention next time