The docker login command can be used to log into docker.io | https://hub.docker.com | Docker Hub.

sudo docker login docker.io

Optionally, the -u or --username and -p or --password options can be used.

docker login --username john.doe --password itsasecret docker.io

If login is successful, something like this should be displayed.

~]$ sudo docker login --username john.doe docker.io
Password:
Login Succeeded