image.yaml: use zstd compression

We'll experiment with this in `rawhide` to see what kind of real
world gains we get from using zstd compression. Also see if there
are any bugs that crop up. This is to further the discussion in
coreos/fedora-coreos-tracker#1660

image.yaml

@@ -9,3 +9,7 @@ include: image-base.yaml
 # and enable it by default for rawhide!
 deploy-via-container: true
 container-imgref: "ostree-remote-registry:fedora:quay.io/fedora/fedora-coreos:rawhide"
+
+# Experiment with using zstd compression in rawhide
+compressor: zstd
+