Using Docker to control environment is great, but one does not want all build tools, source code and test results to be part of production image. So the need for a multi-step docker build (see https://github.com/docker/docker/issues/7115)

I use to have two Dockerfiles, one to build form source, the second to package binary into production environment. In the middle I need to get binaries from the build image to create second docker build context. As `docker cp` does not support copy from image (https://github.com/docker/docker/issues/7710) I have to create a container, copy, delete container.

Would be nice for docker-pipeline to make this simple by offering a `cp` method on image.