Do not warn a user regarding platform support if they are targeting a specific platform in the Dockerfile

[toochevere]

Detect whether the user is selecting the image with a sha256 digest (as support agents have been advising users to do for years, as a workaround for lack of multiarch support), or using FROM --platform. In such cases, do not warn the user if there is a mix of images from manifest lists versus single arch.

Alternatively, this warning could be removed already and we could inject the appropriate --platform info in the FROM line of the Docker file stream.

See build.ts.

See comments in: #96