docker auth config support

[crazy-max]

cc @cdupuis

[cdupuis]

Awesome 🚀 Thank you.

[huapox]

Great, usable for me:

headless @ barge in ~ |18:07:22  
$ cat .docker/config.json 
{
	"auths": {
		"test.registry.ssl:8143": {
			"auth": "XXX="
		},
        "172.25.20.161:18443": {
			"auth": "XXX="
		}
	},
	"HttpHeaders": {
		"User-Agent": "Docker-Client/18.09.8 (linux)"
	}
}
``