if docker login worked, you will find a .docker folder on your home directory (~/.docker/), with a config.json file with credentials in it.
otherwise you would get an error login in.
Note: docker determine what credentials to use by looking at the registry name:
if you do
docker pull myregistry.com/myimage:tag
docker will look if you’re logged in, and if not will check if you have the credentials for the registry myregistry.com and login with those.
If not you will get a permission error