Support targeting `scratch` as a base image

[baronfel]

Even though .NET doesn't support truly static compilation, it's possible that someone might want to package their app in a single standalone layer. The common way to do this is to use the scratch base image. However, today we don't support this - the user must jump through hoops:

• they have to specify a label on the base image (latest)
• they get an error communicating with docker.io

We should add support for scratch base image use:

• [ ] don't require tags - default to latest if no tag is provided on a ContainerBaseImage
• [ ] if the base repository name is scratch, create a default Image that has the minimal required structures to be viable.