+from unittest import mock
+
 from docker.errors import NotFound

 from testcontainers.core.container import DockerContainer
 from testcontainers.core.docker_client import DockerClient
 from testcontainers.core.waiting_utils import wait_container_is_ready
+from testcontainers.core.config import testcontainers_config as c

 from testcontainers.registry import DockerRegistryContainer

@@ -31,8 +34,7 @@ def test_missing_on_private_registry(monkeypatch):
         # prepare auth config
         creds: bytes = base64.b64encode(f"{username}:{password}".encode("utf-8"))
         config = {"auths": {f"{registry_url}": {"auth": creds.decode("utf-8")}}}
-        monkeypatch.setenv("DOCKER_AUTH_CONFIG", json.dumps(config))
-        assert os.environ.get("DOCKER_AUTH_CONFIG"), "DOCKER_AUTH_CONFIG not set"
+        mock.patch.object(c, "_docker_auth_config", json.dumps(config))