some reordering and test optimizations to align new API.

src/ContainerRuntime/ContainerRuntimeClient.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Testcontainers\ContainerRuntime;
+
+use Docker\Docker as DockerClient;
+
+class ContainerRuntimeClient
+{
+    /**
+     * @var DockerClient|null Singleton instance of DockerClient
+     */
+    private static ?DockerClient $dockerClient = null;
+
+    /**
+     * Private constructor to prevent creating instance outside the class.
+     */
+    private function __construct()
+    {
+    }
+
+    /**
+     * Returns the singleton DockerClient instance.
+     *
+     * @return DockerClient The singleton DockerClient instance.
+     * @throws \RuntimeException If the DockerClient instance could not be created.
+     */
+    public static function getDockerClient(): DockerClient
+    {
+        if (self::$dockerClient === null) {
+            self::$dockerClient = DockerClient::create();
+        }
+
+        return self::$dockerClient;
+    }
+
+    /**
+     * Injects a DockerClient instance for testing or special use cases.
+     *
+     * @param DockerClient $client The DockerClient instance to set.
+     */
+    public static function setDockerClient(DockerClient $client): void
+    {
+        self::$dockerClient = $client;
+    }
+}