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/vendor/symfony/mime/Part/AbstractPart.php

@@ -0,0 +1,65 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Part;
+
+use Symfony\Component\Mime\Header\Headers;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+abstract class AbstractPart
+{
+    private Headers $headers;
+
+    public function __construct()
+    {
+        $this->headers = new Headers();
+    }
+
+    public function getHeaders(): Headers
+    {
+        return $this->headers;
+    }
+
+    public function getPreparedHeaders(): Headers
+    {
+        $headers = clone $this->headers;
+        $headers->setHeaderBody('Parameterized', 'Content-Type', $this->getMediaType().'/'.$this->getMediaSubtype());
+
+        return $headers;
+    }
+
+    public function toString(): string
+    {
+        return $this->getPreparedHeaders()->toString()."\r\n".$this->bodyToString();
+    }
+
+    public function toIterable(): iterable
+    {
+        yield $this->getPreparedHeaders()->toString();
+        yield "\r\n";
+        yield from $this->bodyToIterable();
+    }
+
+    public function asDebugString(): string
+    {
+        return $this->getMediaType().'/'.$this->getMediaSubtype();
+    }
+
+    abstract public function bodyToString(): string;
+
+    abstract public function bodyToIterable(): iterable;
+
+    abstract public function getMediaType(): string;
+
+    abstract public function getMediaSubtype(): string;
+}