Add post-extract plugin trigger

This trigger allows you to modify the contents of an application *after* it has been extracted from git/tarball but *before* the image source type is detected. For example, you may wish to autogenerate a Dockerfile based on some other application contents before detecting what type of build process - herokuish or docker file - to use.

docs/development/plugin-triggers.md

@@ -481,6 +481,28 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 sudo service haproxy reload
 ```
 
+### `post-extract`
+
+- Description: Allows you to modify the contents of an application *after* it has been extracted from git/tarball but *before* the image source type is detected.
+- Invoked by: `dokku tar:in`, `dokku tar:from` and the `receive-app` plugin trigger
+- Arguments: `$APP` `$TMP_WORK_DIR` `$REV`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+# Adds a clock process to an app's Procfile
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+APP="$1"; verify_app_name "$APP"
+TMP_WORK_DIR="$2"
+REV="$3" # optional, may not be sent for tar-based builds
+
+pushd "$TMP_WORK_DIR" > /dev/null
+touch Procfile
+echo "clock: some-command" >> Procfile
+```
+
 ### `post-proxy-ports-update`
 
 - Description: Allows you to run commands once the proxy port mappings for an application have been updated. It also sends the invoking command. This can be "add", "clear" or "remove".