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/league/commonmark/src/Reference/ReferenceMap.php

@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the league/commonmark package.
+ *
+ * (c) Colin O'Dell <colinodell@gmail.com>
+ *
+ * Original code based on the CommonMark JS reference parser (https://bitly.com/commonmark-js)
+ *  - (c) John MacFarlane
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace League\CommonMark\Reference;
+
+use League\CommonMark\Normalizer\TextNormalizer;
+
+/**
+ * A collection of references, indexed by label
+ */
+final class ReferenceMap implements ReferenceMapInterface
+{
+    /** @psalm-readonly */
+    private TextNormalizer $normalizer;
+
+    /**
+     * @var array<string, ReferenceInterface>
+     *
+     * @psalm-readonly-allow-private-mutation
+     */
+    private array $references = [];
+
+    public function __construct()
+    {
+        $this->normalizer = new TextNormalizer();
+    }
+
+    public function add(ReferenceInterface $reference): void
+    {
+        // Normalize the key
+        $key = $this->normalizer->normalize($reference->getLabel());
+        // Store the reference
+        $this->references[$key] = $reference;
+    }
+
+    public function contains(string $label): bool
+    {
+        $label = $this->normalizer->normalize($label);
+
+        return isset($this->references[$label]);
+    }
+
+    public function get(string $label): ?ReferenceInterface
+    {
+        $label = $this->normalizer->normalize($label);
+
+        return $this->references[$label] ?? null;
+    }
+
+    /**
+     * @return \Traversable<string, ReferenceInterface>
+     */
+    public function getIterator(): \Traversable
+    {
+        foreach ($this->references as $normalizedLabel => $reference) {
+            yield $normalizedLabel => $reference;
+        }
+    }
+
+    public function count(): int
+    {
+        return \count($this->references);
+    }
+}