Log into a docker registry.
# To interactively log into a registry:
docker login
# To log into a registry with a specific username (user will be prompted for a password):
docker login --username <username>
# To log into a registry with username and password:
docker login --username <username> --password <password> <server>
# To log into a registry with password from stdin:
echo "<password>" | docker login --username <username> --password-stdin