docker pull error when do not add domain prefix

This is no error when:

```bash
docker pull xxx.xxx.com/library/nginx
```

but it does not work when:

```bash
docker pull nginx
````

I have been write docker config

```bash
root@k8s-worker1:~# cat /etc/docker/daemon.json
{
"registry-mirrors": ["https://xxx.xxx.com"],
"exec-opts": ["native.cgroupdriver=systemd"]
}
```

and, I verify it at docker info:

```bash
root@k8s-worker1:~# docker info | tail
Docker Root Dir: /var/lib/docker
Debug Mode: false
Experimental: false
Insecure Registries:
::1/128
127.0.0.0/8
Registry Mirrors:
https://xxx.xxx.com/
Live Restore Enabled: false
```

so how can i pull image without add the domain prefix when i pull it.