The DOKKU_APP_TYPE has long since ceased to be the correct way to specify the builder for the application. It's only usage has been during the detection phase, specifically to ensure that the herokuish plugin injects the correct docker arguments during the build. As such, it is safe to migrate away to a property in a patch release.
Users that seek to set a specific builder should use 'dokku builder:set $APP selected' instead.
Refs #7863
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.
