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-herokuish/builder-release

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/config/functions"
+
+trigger-builder-release-builder-herokuish() {
+  declare desc="builder-herokuish builder-release plugin trigger"
+  declare trigger="trigger-builder-release-builder-herokuish"
+  declare BUILDER_TYPE="$1" APP="$2" IMAGE_TAG="$3"
+
+  if [[ "$BUILDER_TYPE" != "herokuish" ]]; then
+    return
+  fi
+
+  local CID
+  local IMAGE=$(get_app_image_name "$APP" "$IMAGE_TAG")
+  local DOCKER_COMMIT_LABEL_ARGS=("--change" "LABEL org.label-schema.schema-version=1.0" "--change" "LABEL org.label-schema.vendor=dokku" "--change" "LABEL com.dokku.app-name=$APP")
+  local DOCKER_RUN_LABEL_ARGS="--label=com.dokku.app-name=$APP"
+
+  plugn trigger pre-release-buildpack "$APP" "$IMAGE_TAG"
+  if [[ -n $(config_export global) ]]; then
+    CID=$(config_export global | "$DOCKER_BIN" run "${DOCKER_RUN_LABEL_ARGS[@]}" $DOKKU_GLOBAL_RUN_ARGS -i -a stdin "$IMAGE" /bin/bash -c "mkdir -p /app/.profile.d && cat > /app/.profile.d/00-global-env.sh")
+    test "$("$DOCKER_BIN" wait "$CID")" -eq 0
+
+    "$DOCKER_BIN" commit "${DOCKER_COMMIT_LABEL_ARGS[@]}" "$CID" "$IMAGE" >/dev/null
+  fi
+  if [[ -n $(config_export app "$APP") ]]; then
+    CID=$(config_export app "$APP" | "$DOCKER_BIN" run "${DOCKER_RUN_LABEL_ARGS[@]}" $DOKKU_GLOBAL_RUN_ARGS -i -a stdin "$IMAGE" /bin/bash -c "mkdir -p /app/.profile.d && cat > /app/.profile.d/01-app-env.sh")
+    test "$("$DOCKER_BIN" wait "$CID")" -eq 0
+
+    "$DOCKER_BIN" commit "${DOCKER_COMMIT_LABEL_ARGS[@]}" "$CID" "$IMAGE" >/dev/null
+  fi
+  plugn trigger post-release-buildpack "$APP" "$IMAGE_TAG"
+}
+
+trigger-builder-release-builder-herokuish "$@"