[Feature request] Support for docker's multi-stage build about habitus

[mumoshu]

Just for clarification - why would I need two steps? Can't I write build.yml like:

build:
  version: 2016-03-14 # version of the build schema.
  steps:
    runtime:
      name: runtime
      dockerfile: Dockerfile

so that it will trigger a multi-stage build with the exact same output?

The answer is yes, and no.

It works, but am now unable to docker-push the intermediate builder image.

You basically need to run docker build --target foo -t myimg-foo:$VER to tag the intermediate image for docker-push. The docker-push is required in order to use the external docker registry as the global cache of the intermediate image. And it is, IHMO, a common pattern seen in CI/CD env.

[mumoshu]

Submitted #90 for this.