The image "gcr.io/authenticated-image-pulling/windows-nanoserver:v1" is not a
manifest list, and it is only useful for Windows Server 1809, which means that the
test "should be able to pull from private registry with secret" will fail for
environments with Windows Server 1903, 1909, or any other future version we might
want to test.
This commit adds the the ability to have an alternative private image to pull by
using a configurable docker config file which contains the necessary credentials
needed to pull the image.
1a8183d262