Support passing in additional arguments to docker build

[stuarthicks]

I have a project using golang-builder, and I'd like to reference environment variables in my Dockerfile using the ENV directive (docs). Unfortunately, it appears that this still requires the -e/--build-arg params on the call to docker build (docs).

Currently this does not appear to be possible with golang-builder, so I'd like to discuss on this issue possible ways to introduce this feature.

To start things off, how about an environment variable passed in called something like DOCKER_BUILD_ARGS that allows any arbitrary extra args to be given to the docker build command. That seems like it would be a generic-enough implementation. What do you think?