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/Input/MarkdownInput.php

@@ -0,0 +1,102 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the league/commonmark package.
+ *
+ * (c) Colin O'Dell <colinodell@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace League\CommonMark\Input;
+
+use League\CommonMark\Exception\UnexpectedEncodingException;
+
+class MarkdownInput implements MarkdownInputInterface
+{
+    /**
+     * @var array<int, string>|null
+     *
+     * @psalm-readonly-allow-private-mutation
+     */
+    private ?array $lines = null;
+
+    /** @psalm-readonly-allow-private-mutation */
+    private string $content;
+
+    /** @psalm-readonly-allow-private-mutation */
+    private ?int $lineCount = null;
+
+    /** @psalm-readonly */
+    private int $lineOffset;
+
+    public function __construct(string $content, int $lineOffset = 0)
+    {
+        if (! \mb_check_encoding($content, 'UTF-8')) {
+            throw new UnexpectedEncodingException('Unexpected encoding - UTF-8 or ASCII was expected');
+        }
+
+        // Strip any leading UTF-8 BOM
+        if (\substr($content, 0, 3) === "\xEF\xBB\xBF") {
+            $content = \substr($content, 3);
+        }
+
+        $this->content    = $content;
+        $this->lineOffset = $lineOffset;
+    }
+
+    public function getContent(): string
+    {
+        return $this->content;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function getLines(): iterable
+    {
+        $this->splitLinesIfNeeded();
+
+        \assert($this->lines !== null);
+
+        /** @psalm-suppress PossiblyNullIterator */
+        foreach ($this->lines as $i => $line) {
+            yield $this->lineOffset + $i + 1 => $line;
+        }
+    }
+
+    public function getLineCount(): int
+    {
+        $this->splitLinesIfNeeded();
+
+        \assert($this->lineCount !== null);
+
+        return $this->lineCount;
+    }
+
+    private function splitLinesIfNeeded(): void
+    {
+        if ($this->lines !== null) {
+            return;
+        }
+
+        $lines = \preg_split('/\r\n|\n|\r/', $this->content);
+        if ($lines === false) {
+            throw new UnexpectedEncodingException('Failed to split Markdown content by line');
+        }
+
+        $this->lines = $lines;
+
+        // Remove any newline which appears at the very end of the string.
+        // We've already split the document by newlines, so we can simply drop
+        // any empty element which appears on the end.
+        if (\end($this->lines) === '') {
+            \array_pop($this->lines);
+        }
+
+        $this->lineCount = \count($this->lines);
+    }
+}

docker/streamline-src/vendor/league/commonmark/src/Input/MarkdownInputInterface.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the league/commonmark package.
+ *
+ * (c) Colin O'Dell <colinodell@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace League\CommonMark\Input;
+
+interface MarkdownInputInterface
+{
+    public function getContent(): string;
+
+    /**
+     * @return iterable<int, string>
+     */
+    public function getLines(): iterable;
+
+    public function getLineCount(): int;
+}