ecr: Fix "imageDigest" value in ecr.list_images() response (#3436)

moto/ecr/models.py

@@ -164,7 +164,7 @@ class Image(BaseObject):
     def response_list_object(self):
         response_object = self.gen_response_object()
         response_object["imageTag"] = self.image_tag
-        response_object["imageDigest"] = "i don't know"
+        response_object["imageDigest"] = self.get_image_digest()
         return {
             k: v for k, v in response_object.items() if v is not None and v != [None]
         }