Added authentication service for docker registry

This adds a simple authentication service for docker which uses current user credentials to authenticate pulls and pushes.

I have only one concern. Since the `.docker/config` is unencrypted, thus the password for user stored there is unencrypted, maybe we should from the start implement function to generate/provide a separate password just for the purposes of accessing docker registry?

What do you think @jacobvosmaer @sytses @marin?

cc @marin 

See merge request !3787