diff --git a/pre_commit/languages/docker.py b/pre_commit/languages/docker.py
index cb29a021d..b54ceeca0 100644
--- a/pre_commit/languages/docker.py
+++ b/pre_commit/languages/docker.py
@@ -28,7 +28,7 @@ def _is_in_docker() -> bool:
 def _get_docker_path(path: str) -> str:
     if not _is_in_docker():
         return path
-    with open('/proc/1/cgroup', 'r') as f:
+    with open('/proc/1/cgroup') as f:
         container_id = os.path.basename(f.readlines()[0]).strip()
 
     _, out, _ = cmd_output_b('docker', 'inspect', container_id)