Add support for handling the --platform flag in FROM

[rcjsuen]

The --platform flag was added to FROM instructions in Docker CE 18.04. I wasn't really aware of this as I relied on the documentation to keep up-to-date on the latest changes to the Dockerfile specification.

Will need to ensure this is no longer flagged as a warning and then to also update our version support accordingly.

dockerfile-utils/bin/dockerfile-utils

Line 44 in d6912d8

console.log("dockerfile-utils version " + package.version + " (supports Docker CE 18.03)");