Add is_restarting in docker module (#678)

Author: MahdiAbbasi95

testinfra/modules/docker.py

@@ -41,6 +41,10 @@ def inspect(self):
     def is_running(self):
         return self.inspect()["State"]["Running"]
 
+    @property
+    def is_restarting(self):
+        return self.inspect()["State"]["Restarting"]
+
     @property
     def id(self):
         return self.inspect()["Id"]