docker login addes an extra registry setting to .docker/config.json

Issue

• When I executes docker lgoin, my ~/.docker/config.json always contains some one extra registry.

• For example, when I login to docker.io with

# docker login docker.io

• Then, my ~/.docker/config.json is configured as:

{
    "auths": {
        "https://index.docker.io/v1/": {
            "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
            "email": "foo@example.com"
        },
        "registry.hub.docker.com": {
            "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
            "email": "bar@example.com"
        }
    }
}

• Why "registry.hub.docker.com" with email has been added?