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/Header/UnstructuredHeader.php

@@ -0,0 +1,70 @@
+<?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\Header;
+
+/**
+ * A Simple MIME Header.
+ *
+ * @author Chris Corbyn
+ */
+class UnstructuredHeader extends AbstractHeader
+{
+    private string $value;
+
+    public function __construct(string $name, string $value)
+    {
+        parent::__construct($name);
+
+        $this->setValue($value);
+    }
+
+    /**
+     * @param string $body
+     *
+     * @return void
+     */
+    public function setBody(mixed $body)
+    {
+        $this->setValue($body);
+    }
+
+    public function getBody(): string
+    {
+        return $this->getValue();
+    }
+
+    /**
+     * Get the (unencoded) value of this header.
+     */
+    public function getValue(): string
+    {
+        return $this->value;
+    }
+
+    /**
+     * Set the (unencoded) value of this header.
+     *
+     * @return void
+     */
+    public function setValue(string $value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * Get the value of this header prepared for rendering.
+     */
+    public function getBodyAsString(): string
+    {
+        return $this->encodeWords($this, $this->value);
+    }
+}