Enable composefs root

This partially reverts commit 7977ead6e4 and
effectively migrates the change from 8f5be09371
and 14ab1c5847
to here.

Signed-off-by: Colin Walters <walters@verbum.org>

copr-walters-fasttracks.repo

@@ -8,4 +8,3 @@ gpgkey=https://download.copr.fedorainfracloud.org/results/walters/fedora-bootc-f
 repo_gpgcheck=0
 enabled=1
 enabled_metadata=1
-

tier-0/ostree.yaml

@@ -7,9 +7,8 @@ postprocess:
     #!/usr/bin/env bash
     mkdir -p /usr/lib/ostree
     cat > /usr/lib/ostree/prepare-root.conf << EOF
-    # TODO disabled due to https://github.com/osbuild/bootc-image-builder/issues/149
-    #[root]
-    #transient = true
+    [composefs]
+    enabled = true
     [sysroot]
     readonly = true
     EOF