How can use nexus as mirror registry for docker

I want use nexus as mirror registry in my docker . Actually I want when I am perform pull and image from docker hub it could also reside image in nexus repository. Now I can pull image from docker hub but a copy from the image does not reside on nexus repo .

Actually I did follow steps :
1- create repository (proxy)
2- add http port = 8123
3- enable docker api/v1
4-remote storage ; select use proxy registry and use follow address:

http://172.20.20.20 (this is nexus ip addres)
and other options are default

finally add : http://172.20.20.20:8123
in docker registry daemon as mirror registry
Now after pull an image it just resides on docker host and cannot reside any copy from image on nexus repo

How can solve the issue ?