Enable check mode for docker_image_facts (#38944)

This module never changes anything. As such, there is no reason not to
just enable check mode.

lib/ansible/modules/cloud/docker/docker_image_facts.py

@@ -218,7 +218,8 @@ def main():
     )
 
     client = AnsibleDockerClient(
-        argument_spec=argument_spec
+        argument_spec=argument_spec,
+        supports_check_mode=True
     )
 
     results = dict(