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/sebastian/code-unit/src/CodeUnitCollectionIterator.php

@@ -0,0 +1,54 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of sebastian/code-unit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace SebastianBergmann\CodeUnit;
+
+use Iterator;
+
+/**
+ * @template-implements Iterator<int, CodeUnit>
+ */
+final class CodeUnitCollectionIterator implements Iterator
+{
+    /**
+     * @psalm-var list<CodeUnit>
+     */
+    private array $codeUnits;
+    private int $position = 0;
+
+    public function __construct(CodeUnitCollection $collection)
+    {
+        $this->codeUnits = $collection->asArray();
+    }
+
+    public function rewind(): void
+    {
+        $this->position = 0;
+    }
+
+    public function valid(): bool
+    {
+        return isset($this->codeUnits[$this->position]);
+    }
+
+    public function key(): int
+    {
+        return $this->position;
+    }
+
+    public function current(): CodeUnit
+    {
+        return $this->codeUnits[$this->position];
+    }
+
+    public function next(): void
+    {
+        $this->position++;
+    }
+}