refactor: add initial support for builder plugins

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.

docs/development/plugin-triggers.md

@@ -68,6 +68,36 @@ case "$URL_TYPE" in
 esac
 ```
 
+### `builder-build`
+
+- Description: Triggers the artifact build process
+- Invoked by: `dokku deploy`
+- Arguments: `$BUILDER_TYPE` `$APP` `$SOURCECODE_WORK_DIR`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
+### `builder-release`
+
+- Description: Triggers the artifact release process
+- Invoked by: `dokku deploy`
+- Arguments: `$BUILDER_TYPE` `$APP` `$IMAGE_TAG`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `check-deploy`
 
 - Description: Allows you to run checks on a deploy before Dokku allows the container to handle requests.