diff --git a/slowapi/extension.py b/slowapi/extension.py
index 050f882..c04ff4a 100644
--- a/slowapi/extension.py
+++ b/slowapi/extension.py
@@ -87,6 +87,9 @@ def _rate_limit_exceeded_handler(request: Request, exc: RateLimitExceeded) -> Re
     return response
 
 
+_UNSET = object()  # sentinel: distinguishes "not passed" from explicit None
+
+
 class Limiter:
     """
     Initializes the slowapi rate limiter.
@@ -123,7 +126,10 @@ class Limiter:
     * **key_prefix**: prefix prepended to rate limiter keys.
     * **enabled**: set to False to deactivate the limiter (default: True)
     * **config_filename**: name of the config file for Starlette from which to load settings
-     for the rate limiter. Defaults to ".env".
+     for the rate limiter. When not specified, auto-detects ".env" in the current directory
+     and loads it if found. Pass ``None`` explicitly to disable all file loading (recommended
+     for Docker and Kubernetes deployments where environment variables are injected by the
+     orchestration layer rather than from a file).
     * **key_style**: set to "url" to use the url, "endpoint" to use the view_func
     """
 
@@ -144,7 +150,7 @@ def __init__(
         retry_after: Optional[str] = None,
         key_prefix: str = "",
         enabled: bool = True,
-        config_filename: Optional[str] = None,
+        config_filename: Optional[str] = _UNSET,  # type: ignore[assignment]
         key_style: Literal["endpoint", "url"] = "url",
     ) -> None:
         """
@@ -155,13 +161,14 @@ def __init__(
         # app.state.limiter = self
 
         self.logger = logging.getLogger("slowapi")
-
-        dotenv_file_exists = os.path.isfile(".env")
-        self.app_config = Config(
-            ".env"
-            if dotenv_file_exists and config_filename is None
-            else config_filename
-        )
+        if config_filename is _UNSET:
+            # Backward-compatible auto-detect: load .env only if it exists
+            _config_file: Optional[str] = ".env" if os.path.isfile(".env") else None
+        else:
+            # User explicitly passed config_filename.
+            # None means "don't load any file" (e.g. Docker/K8s envs).
+            _config_file = config_filename  # type: ignore[assignment]
+        self.app_config = Config(_config_file)
 
         self.enabled = enabled
         self._default_limits = []
diff --git a/tests/test_fastapi_extension.py b/tests/test_fastapi_extension.py
index 42e6322..387c94f 100644
--- a/tests/test_fastapi_extension.py
+++ b/tests/test_fastapi_extension.py
@@ -4,7 +4,8 @@
 from starlette.responses import PlainTextResponse, Response
 from starlette.testclient import TestClient
 
-from slowapi.util import get_ipaddr
+from slowapi.extension import Limiter
+from slowapi.util import get_ipaddr, get_remote_address
 from tests import TestSlowapi
 
 
@@ -369,3 +370,58 @@ async def t1_func(my_param: str, request: Request):
                 )
                 == 2
             )
+
+
+# --------------------------------------------------------------------------
+# Tests for config_filename sentinel behaviour (issue #256)
+# --------------------------------------------------------------------------
+
+def test_config_filename_none_disables_dotenv_loading(tmp_path, monkeypatch):
+    """
+    Passing config_filename=None explicitly must NOT load a .env file,
+    even if one exists in the current directory.
+    This is the Docker/K8s opt-out: the user manages their own env vars.
+    """
+    env_file = tmp_path / ".env"
+    env_file.write_text("RATELIMIT_TEST_SENTINEL=loaded_from_file\n")
+    monkeypatch.chdir(tmp_path)  # make os.path.isfile(".env") see the file
+
+    limiter = Limiter(key_func=get_remote_address, config_filename=None)
+
+    assert (
+        limiter.app_config("RATELIMIT_TEST_SENTINEL", default="not_loaded")
+        == "not_loaded"
+    ), "config_filename=None should disable .env loading"
+
+
+def test_config_filename_unset_loads_dotenv_when_present(tmp_path, monkeypatch):
+    """
+    Omitting config_filename entirely should preserve the existing behaviour:
+    auto-detect and load .env when it exists in the current directory.
+    """
+    env_file = tmp_path / ".env"
+    env_file.write_text("RATELIMIT_TEST_SENTINEL=loaded_from_file\n")
+    monkeypatch.chdir(tmp_path)
+
+    limiter = Limiter(key_func=get_remote_address)
+
+    assert (
+        limiter.app_config("RATELIMIT_TEST_SENTINEL", default="not_loaded")
+        == "loaded_from_file"
+    ), "omitting config_filename should auto-load .env when present"
+
+
+def test_config_filename_unset_no_dotenv_does_not_error(tmp_path, monkeypatch):
+    """
+    Omitting config_filename when no .env file exists must not raise.
+    This covers the original issue: containers without a .env file mounted.
+    """
+    monkeypatch.chdir(tmp_path)  # tmp_path is empty, no .env present
+
+    # Must not raise any exception
+    limiter = Limiter(key_func=get_remote_address)
+
+    assert (
+        limiter.app_config("RATELIMIT_TEST_SENTINEL", default="fallback")
+        == "fallback"
+    )