The IMAGE_TAG is not always available, while the IMAGE_SOURCE_TYPE is. Since the point of these triggers was to unify the argument passed and make it easier to interact with them, this version makes more sense.
Closes#3780
This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.
This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.
A builder plugin implements some method for 'building' artifacts from a given source code archive. This refactor moves the majority of the image building code into separate plugins, allowing us to implement other builders, but also simplifying the common pugin codebase.
Also move the pre-build-buildpack hook from the build-env plugin to builder-herokuish.
