support for build-args (and ARG)?

[jrandall]

I think there is currently no way to pass build arguments to the build (as is done with docker build --build-arg FOO=bar). The main use case I have for this is to build FROM a base image that has the same build version as the one I am currently building.

E.g. a dockerfile starting with:

ARG  VERSION=latest
FROM base:${VERSION}

In order to support this, a new build config field would need to support passing of build args, and they'd have to be passed to docker build as well as handled in Dockerfile ARG lines.