This fixes an issue where disk space is not recouped after deploys unless 'dokku cleanup' without arguments is called.
Also injects the label-schema labels as necessary.
Refs #4099
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.
