docker compose exemplified, .env integration

public/index.php

@@ -9,9 +9,14 @@ use Slim\Factory\AppFactory;
 
 require __DIR__ . '/../vendor/autoload.php';
 
-// Load environment variables
-$dotenv = Dotenv\Dotenv::createImmutable(__DIR__ . '/../');
-$dotenv->load();
+// Load environment variables when a project .env file is present.
+// Docker can provide these via env_file/environment as well; in that case
+// we should not crash if the file is not mounted into the container.
+$dotenvPath = __DIR__ . '/../';
+if (is_file($dotenvPath . '.env')) {
+    $dotenv = Dotenv\Dotenv::createImmutable($dotenvPath);
+    $dotenv->safeLoad();
+}
 
 // Build DI container
 $containerBuilder = new ContainerBuilder();