Add support for private container registries

[mwiens]

Problem

konlet currently does not support starting images from private registries that need authentication (username and token).

When providing in gce-container-declaration metadata the following

"image": "registry.gitlab.mysite.com/my-project/my-software-container:latest"

to run my container on the GCE with COS, I get following message (from sudo journalctl -u konlet-startup):

May 14 15:40:38 test-vm konlet-startup[35691]: 2020/05/14 15:40:38 Error: Failed to start container:Error response from daemon: {"message":"Get https://registry.gitlab.mysite.com/my-project/my-software-container/manifests/latest: denied: access forbidden"}

But the same script works with "image": "busybox".

Proposal

Add additional parameters like registry-auth-user, registry-auth-token and maybe registry-auth-url. When the used registry is not one from GCP, perform echo -n registry-auth-token | docker login -u registry-auth-user --password-stdin registry-auth-url or similar, so the corresponding container can be pulled.

Thank you.