A more reliable way to get the container id.

The hostname is not always the container id. Get the container id from
/proc/1/cgroup. Fixes #1918.

pre_commit/languages/docker.py

@@ -1,7 +1,6 @@
 import hashlib
 import json
 import os
-import socket
 from typing import Sequence
 from typing import Tuple
 
@@ -29,9 +28,11 @@ def _is_in_docker() -> bool:
 def _get_docker_path(path: str) -> str:
     if not _is_in_docker():
         return path
-    hostname = socket.gethostname()
+    container_id = os.path.basename(
+        open('/proc/1/cgroup').readlines()[0],
+    ).strip()
 
-    _, out, _ = cmd_output_b('docker', 'inspect', hostname)
+    _, out, _ = cmd_output_b('docker', 'inspect', container_id)
 
     container, = json.loads(out)
     for mount in container['Mounts']: