With this implement, we can add variables at building stage.
For example, add HTTP_PROXY and NO_PROXY when needed like below.

    build.py --build-args \
        HTTP_PROXY:http://127.0.0.1:8080,NO_PROXY:127.0.0.1

More info about build_arg, pls check[0]

[0] https://docs.docker.com/engine/reference/commandline/build/#set-build-time-variables-build-arg

DocImpact

Implements: bp add-buildargs
Change-Id: I29ed7f536670fef59d697603dc562a69d90743c9