why can't map local path to docker's inside? I want to build private docker hub. and I did it with:

 docker run -it --restart=always -p 7901:5000   -v /data/registry:/tmp/registry registry /bash/bin


And my disk informations as below:

[root@10-19-47-24 docker]# df -lh
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        20G  4.4G   16G  22% /
devtmpfs        3.9G     0  3.9G   0% /dev
tmpfs           3.9G     0  3.9G   0% /dev/shm
tmpfs           3.9G  401M  3.5G  11% /run
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/vdb         60G  572M   60G   1% /data
tmpfs           783M     0  783M   0% /run/user/0
tmpfs           783M     0  783M   0% /run/user/1001
tmpfs           783M     0  783M   0% /run/user/1000
overlay          20G  4.4G   16G  22% /var/lib/docker/overlay2/9923f757c58c0792526f4cb32750401847f127b9e33c0dc78a29d7933bcde5bf/merged
shm              64M     0   64M   0% /var/lib/docker/containers/51959fbdc1663f968eff770d54c56bc04ddaba2c2d3623058c90a00f582fa40c/mounts/shm


And my version of docker is:

[root@10-19-47-24 docker]# docker --version
Docker version 18.06.0-ce, build 0ffa825


And I can push hello-world to the hub, but I can't see anything inside the /data/registry. My question is where is my registry? Is there any mistake of my command?
Oh! I have search for a while, it's should be like:

docker run -d --name=ak_registry  --restart=always -p 7900:5000   -v /data/.docker:/var/lib/registry    registry


Used /var/lib/registry instead of /tmp/registry