Build modified streamline images

The official image from streamline does not currently work for the arm64
platform. As a temporary measure, the source code and docker build scripts
have been lifted from the official images and are used to build locally.

Some additional modifications are made to reduce overall image size, these
are documented in docker/README.md

docker/streamline-src/app/Events/Event.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Streamline\Events;
+
+use Illuminate\Broadcasting\Channel;
+use Illuminate\Queue\SerializesModels;
+use Illuminate\Broadcasting\PrivateChannel;
+use Illuminate\Broadcasting\PresenceChannel;
+use Illuminate\Foundation\Events\Dispatchable;
+use Illuminate\Broadcasting\InteractsWithSockets;
+use Illuminate\Contracts\Broadcasting\ShouldBroadcast;
+
+class Event
+{
+    use Dispatchable, InteractsWithSockets, SerializesModels;
+
+    /**
+     * Create a new event instance.
+     *
+     * @return void
+     */
+    public function __construct()
+    {
+        //
+    }
+
+    /**
+     * Get the channels the event should broadcast on.
+     *
+     * @return \Illuminate\Broadcasting\Channel|array
+     */
+    public function broadcastOn()
+    {
+        return new PrivateChannel('channel-name');
+    }
+}