docker provider: Don't explode when removing an image if it is in use

Output from recent Docker has changed:

    Stderr: Error response from daemon: conflict: unable to delete 0ba49dd235e5 (cannot be forced) - image is being used by running container 250bbe980448

Fixes #7245

plugins/providers/docker/driver.rb

@@ -166,6 +166,7 @@ module VagrantPlugins
         return true
       rescue => e
         return false if e.to_s.include?("is using it")
+        return false if e.to_s.include?("is being used")
         raise if !e.to_s.include?("No such image")
       end