Docker registry mirroring failed, why?

the network of my site is in close network for internet, so *.docker.io can not be reachable.
i construct docker registry to mirror, pushed ubuntu.tar image to registry by manually.
(by create topic policy, i worte xttp, but it’s http)

$ wget -qO- xttp://xxx.xxx.xxx.xxx:5000/v2/_catalog
{“repositories”:[“ubuntu”]}

ok, my registry has ubuntu image.

$ cat /etc/default/docker | grep DOCKER_OPTS
DOCKER_OPTS="–registry-mirror=xttp://xxx.xxx.xxx.xxx:5000"

set DOCKER_OPTS, but it’s not connect to my mirror server like that.

$ sudo docker pull ubuntu:16.04
Error response from daemon: Get https://registry-1.docker.io/v2/: net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)

i just followed https://docs.docker.com/registry/recipes/mirror/#configuring-the-cache.

why daemon try to connect *.docker.io, instead of my xttp://xxx.xxx.xxx.xxx:5000?