How to set Docker Backing Filesystem to XFS?
If you are using the overlay2 storage driver, then Docker never creates filesystems. It is simply creating directories on your existing backing filesystem. If that filesystem is using ext4, that's what you're going to get.
The only situation in which Docker actually creates a new filesystem is when using the devicemapper driver, in which case Docker is carving out chunks from a block storage device and then formatting them for use with the filesystem of your choice.