Allow passing arbitrary build args to docker

[yuvipanda]

In Dockerfiles, you can specify --build-arg to be passed into ARG directives in the Dockerfile. Right now, there's no way to do that when using repo2docker as a library. We should make it possible to do so!

[tmhm]

Please lets we know any progress on this ?

[yuvipanda]

@tmhm as far as I know, none! Probably a good issue for someone new to pick up and build. Maybe you! :)

[vliao-semios]

Without customized --build-arg making this not usable since I'll need to change the docker file to hardcode the arguments. On the other hand, allowing it will make repo2jupyter a wonderful development tool that can build the Jupyter set-up image.

[manics]

Do you have a repo to illustrate the use case, or examples of build-args you'd like to pass?

[anjackson]

I can't speak for the others but I'm subscribed to this issue because I need to be able to run repo2jupyter behind a corporate web proxy. Here's how I do it for Docker Compose: https://github.com/ukwa/crawl-streams/blob/a5c9627272e947141987d233f32fc446d560c70d/docker-compose.yml#L5-L9