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/Encoder/Base64Encoder.php

@@ -0,0 +1,41 @@
+<?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\Encoder;
+
+/**
+ * @author Chris Corbyn
+ */
+class Base64Encoder implements EncoderInterface
+{
+    /**
+     * Takes an unencoded string and produces a Base64 encoded string from it.
+     *
+     * Base64 encoded strings have a maximum line length of 76 characters.
+     * If the first line needs to be shorter, indicate the difference with
+     * $firstLineOffset.
+     */
+    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
+    {
+        if (0 >= $maxLineLength || 76 < $maxLineLength) {
+            $maxLineLength = 76;
+        }
+
+        $encodedString = base64_encode($string);
+        $firstLine = '';
+        if (0 !== $firstLineOffset) {
+            $firstLine = substr($encodedString, 0, $maxLineLength - $firstLineOffset)."\r\n";
+            $encodedString = substr($encodedString, $maxLineLength - $firstLineOffset);
+        }
+
+        return $firstLine.trim(chunk_split($encodedString, $maxLineLength, "\r\n"));
+    }
+}