docker-host: Allow to authenticate against private registry for update-images.sh script

[Schnitzel]

In the docker-host we have an update-images.sh script which automatically pulls new images for a defined organization (this way base images are always up to date).

Some customers are storing base images on private registries instead of public ones, as the script has currently no possibility to authenticate against a private registry it cannot pull new images.

Therefore it would be great if the docker-host pod has a possibility to inject a list of authentication (username/password/registry-host) that will be authenticated first before the images are pulled in the update-images.sh script.