Allow pulling images from private registries

[rubencosta]

What

Make use of the auth present in the default Docker config to pull images allowing K3d to pull images from private registries that have been previously authenticated with the docker login.

Why

K3d uses the docker SDK to pull images which doesn't use the stored registry auth credentials like the Docker cli does. If for example, you have to use a private registry as a proxy for pulling images like the K3s image or you want to have your own k3s image for some reason on a private registry, K3d will not be able to pull them as there's no way to authenticate. A possible workaround would be to pull the image before creating a cluster.

Implications

I don't think this introduces any breaking changes whatsoever. All the existing behaviour should remain the same.

[bluestealth]

Interesting this is almost identical to #1580 but I don't think it handles index.docker.io authentication, FWIW https://github.com/docker/cli/blob/636a4cf2dc225f0fe496ba53caeb2d216448fc4d/cli/command/image/push.go#L112 upstream treats registry.AuthConfig and config.AuthConfig as a mere wrapper.

[rubencosta]

@bluestealth totally missed your PR 🤦‍♂️ . You are right, I guess that's a bit of an edge case which I didn't consider being that images will pull from docker.io by default. Happy to close this one as your implementation is more complete and probably safer as it makes use of reference for name parsing.