You don’t have two 18GB overlay files, that’s a red herring. Docker uses overlayfs and those are just how your existing disk is presented to the container. /dev/vda2 is your disk and is mounted at / - that’s where you should be directing your efforts.
If ./launcher cleanup did nothing then I’m assuming docker image prune (removes dangling images) won’t either. If you’re only using this server for discourse you may need to just check around to make sure there are no large files/folders in your home directory.