rumrunner 0.4.0

Rum Runner is a Rake-based utility for building multi-stage Dockerfiles. Users can pair a multi-stage Dockerfile with a Rumfile that uses a Rake-like DSL to customize each stage's build options and dependencies. The `rum` executable allows users to easily invoke builds, shell-into specific stages for debugging, and export artifacts from built containers. Rum Runner has the following features: * Fully compatible with Rake * Rake-like DSL/CLI that enable simple annotation and execution of builds * Rumfiles are completely defined in standard Ruby syntax, like Rakefiles * Users can chain Docker build stages with prerequisites * Artifacts can be exported from stages * Shell tasks are automatically provided for every stage * Stage, artifact, and shell, steps can be customized