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.

plugins/builder-dockerfile/builder-release

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+trigger-builder-release-builder-dockerfile() {
+  declare desc="builder-dockerfile builder-release plugin trigger"
+  declare trigger="trigger-builder-release-builder-dockerfile"
+  declare BUILDER_TYPE="$1" APP="$2" IMAGE_TAG="$3"
+
+  if [[ "$BUILDER_TYPE" != "herokuish" ]]; then
+    return
+  fi
+
+  # buildstep plugins don't necessarily make sense for dockerfiles. call the new breed!!!
+  plugn trigger pre-release-dockerfile "$APP" "$IMAGE_TAG"
+  plugn trigger post-release-dockerfile "$APP" "$IMAGE_TAG"
+}
+
+trigger-builder-release-builder-dockerfile "$@"