Docker: extend `wp shell` with `reload` command

`reload` helper command should extend `wp shell` to reload WordPress context during the session.

In our Docker setup we can do: `yarn docker:wp shell` to use WP-CLI shell.

Because each `wp shell` session counts as a single request, it causes unexpected situations with WP cache.

It's possible to invalidate cache by entering `wp_cache_flush()` between requests you expect to get cached by WordPress.

Having a simple `reload` to reload the complete WordPress context would be great, though.