Docker.io now supports OAuth2 for their API. In order to set it up:
Register a new application by following the instructions in their website: Register Your Application
Fill Consumer Key and Consumer Secret values in settings:
SOCIAL_AUTH_DOCKER_KEY = ''
SOCIAL_AUTH_DOCKER_SECRET = ''
Add 'social.backends.docker.DockerOAuth2' into your SOCIAL_AUTH_AUTHENTICATION_BACKENDS.