Proposal: Add support for build-time environment variables to the 'build' API

A build-time environment variable gets used only while processing the 'RUN' primitive of a DockerFile. Such a variable is only accessible during 'RUN' and is 'not' persisted with the intermediate and final docker images, thereby addressing the portability concerns of the images generated with 'build'.

This addresses issue #4962
Thanks for reviewing @EronWright. I have fixed the spelling and updated the diff