Docker install: add a functionality plugin to the basic setup

When working in the monorepo, you often have to add code snippets to your install:

- to enable features gated by a filter
- to debug or test code from a third-party point of view
- to customize the behavior of your WordPress installation
- to customize Jetpack features in a way that will stick when you switch branches.

For all those things, I've personally created a little `helper` plugin that I added to my `tools/docker/jetpack-docker-config.yml` file:

```
## My Helper plugin
/Users/jeherve/code/helper: /var/www/html/wp-content/plugins/helper
```

This works very well for me, but that's something that not immediately in reach of someone who just discovered the monorepo, or who only contributes very rarely.

I was wondering if we could develop a simple, empty, plugin that we should ship with the Docker installation, to address such a need. The plugin could then be activated by folks who need it, whenever they need extra functionality on their site.

The plugin in itself shouldn't be more than just a few lines:

```php

Start by inspecting the Docker setup and tools/docker/jetpack-docker-config.yml to see how files are mounted into the WordPress installation. Define where the functionality plugin should live and how it should be included in the basic setup. Done means a fresh Docker install provides the plugin and contributors can activate it for local snippets.

Written by the indexing model from the issue text.