deploy: Write a .ostree.cfs composefs image in the deploy dir

This can be used as a composefs source for the root fs instead of
the checkout by pointing the basedir to /ostree/repo/objects.

We enable ensure_rootfs_dirs which adds the required root dirs
to the image. In particular, this includes /etc which often
isn't in ostree commits in use.

Note: This doesn't yet enable fsverity for the images.

src/libostree/ostree-sysroot-deploy.c

@@ -648,6 +648,20 @@ checkout_deployment_tree (OstreeSysroot     *sysroot,
                                 cancellable, error))
     return FALSE;
 
+  /* Create a composefs image and put in deploy dir as .ostree.cfs */
+  OstreeRepoCheckoutComposefsOptions composefs_opts = { 0 };
+  g_autoptr(OstreeComposefsTarget) target = ostree_composefs_target_new ();
+
+  composefs_opts.ensure_rootfs_dirs = TRUE;
+  if (!ostree_repo_checkout_composefs (repo, &composefs_opts, target, csum,
+                                       cancellable, error))
+    return FALSE;
+
+  g_autofree char *composefs_cfs_path = g_strdup_printf("%s/.ostree.cfs", checkout_target_name);
+  if (!ostree_composefs_target_write_at (target, osdeploy_dfd, composefs_cfs_path, NULL,
+                                         cancellable, error))
+    return FALSE;
+
   return glnx_opendirat (osdeploy_dfd, checkout_target_name, TRUE, out_deployment_dfd,
                          error);
 }