diff --git a/public/index.php b/public/index.php
index ccef715..a318860 100644
--- a/public/index.php
+++ b/public/index.php
@@ -9,11 +9,21 @@ use Slim\Factory\AppFactory;
 
 require __DIR__ . '/../vendor/autoload.php';
 
-// 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.
+// Only load the project .env file when the runtime environment has not already
+// provided the required values. Docker commonly sets these via env_file or
+// environment entries, so we must not try to read a missing local .env file in that case.
 $dotenvPath = __DIR__ . '/../';
-if (is_file($dotenvPath . '.env')) {
+$requiredEnvKeys = ['APP_ENV', 'APP_DEBUG', 'LOG_LEVEL', 'PORT', 'RATE_LIMIT', 'CORS_ORIGIN'];
+$hasRuntimeEnvironment = false;
+
+foreach ($requiredEnvKeys as $key) {
+    if (array_key_exists($key, $_ENV) || getenv($key) !== false) {
+        $hasRuntimeEnvironment = true;
+        break;
+    }
+}
+
+if (!$hasRuntimeEnvironment && is_file($dotenvPath . '.env')) {
     $dotenv = Dotenv\Dotenv::createImmutable($dotenvPath);
     $dotenv->safeLoad();
 }