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'

Signed-off-by: Madhav Puri <madhav.puri@gmail.com>