From 77bb6dfe9f2934683f251df820e3609f74eeeaff Mon Sep 17 00:00:00 2001
From: Dean Chen <862469039@qq.com>
Date: Thu, 8 Oct 2026 07:35:21 +0500
Subject: [PATCH] build: match dockerignore names case-sensitively

fnmatch() lowercased both sides on every OS, so a README.md pattern
also dropped readme.md while packing a build context. Use
os.path.normcase, which is what this helper already documented.
Windows still folds case.

Fixes #3450

Signed-off-by: Dean Chen <862469039@qq.com>
---
 docker/utils/fnmatch.py        | 5 +++--
 tests/unit/utils_build_test.py | 9 +++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/docker/utils/fnmatch.py b/docker/utils/fnmatch.py
index be745381e4..31bc8c6993 100644
--- a/docker/utils/fnmatch.py
+++ b/docker/utils/fnmatch.py
@@ -10,6 +10,7 @@
 corresponding to PATTERN.  (It does not compile it.)
 """
 
+import os
 import re
 
 __all__ = ["fnmatch", "fnmatchcase", "translate"]
@@ -39,8 +40,8 @@ def fnmatch(name, pat):
     If you don't want this, use fnmatchcase(FILENAME, PATTERN).
     """
 
-    name = name.lower()
-    pat = pat.lower()
+    name = os.path.normcase(name)
+    pat = os.path.normcase(pat)
     return fnmatchcase(name, pat)
 
 
diff --git a/tests/unit/utils_build_test.py b/tests/unit/utils_build_test.py
index 2089afb49d..aa28e59fc8 100644
--- a/tests/unit/utils_build_test.py
+++ b/tests/unit/utils_build_test.py
@@ -119,6 +119,15 @@ def test_single_filename(self):
             self.all_paths - {'a.py'}
         )
 
+    @pytest.mark.skipif(
+        IS_WINDOWS_PLATFORM, reason='Windows pattern matching folds case'
+    )
+    def test_filename_pattern_is_case_sensitive(self):
+        from docker.utils.build import Pattern
+
+        assert Pattern('README.md').match('README.md')
+        assert not Pattern('README.md').match('readme.md')
+
     def test_single_filename_leading_dot_slash(self):
         assert self.exclude(['./a.py']) == convert_paths(
             self.all_paths - {'a.py'}