Leading slash in .dockerignore should be ignored

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/utils/build.py

@@ -26,6 +26,7 @@ def exclude_paths(root, patterns, dockerfile=None):
     if dockerfile is None:
         dockerfile = 'Dockerfile'
 
+    patterns = [p.lstrip('/') for p in patterns]
     exceptions = [p for p in patterns if p.startswith('!')]
 
     include_patterns = [p[1:] for p in exceptions]

tests/unit/utils_test.py

@@ -768,6 +768,11 @@ class ExcludePathsTest(unittest.TestCase):
             self.all_paths - set(['foo/a.py'])
         )
 
+    def test_single_subdir_single_filename_leading_slash(self):
+        assert self.exclude(['/foo/a.py']) == convert_paths(
+            self.all_paths - set(['foo/a.py'])
+        )
+
     def test_single_subdir_with_path_traversal(self):
         assert self.exclude(['foo/whoops/../a.py']) == convert_paths(
             self.all_paths - set(['foo/a.py'])