diff --git a/README.md b/README.md
index 965405d..24c9138 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,14 @@ print(machineid.hashed_id('myappid'))
 print(machineid.hashed_id())
 ```
 
+## Testing
+
+To run tests, invoke `unittest`:
+
+```bash
+python3 -m unittest
+```
+
 ## Thanks
 
 Special thanks to Denis Brodbeck for his Go package, [`machineid`](https://github.com/denisbrodbeck/machineid).
diff --git a/machineid/__init__.py b/machineid/__init__.py
index 7bf12c1..4933930 100644
--- a/machineid/__init__.py
+++ b/machineid/__init__.py
@@ -86,7 +86,9 @@ def id(winregistry: bool = True) -> str:
       mountinfo = __read__('/proc/self/mountinfo')
       if mountinfo:
         if 'docker' in mountinfo:
-          id = __exec__("grep 'systemd' /proc/self/mountinfo | cut -d/ -f3")
+          id = __exec__("grep '/data/docker/containers' /proc/self/mountinfo | cut -d/ -f5")
+          if not id:
+            id = __exec__("grep '/var/lib/docker/containers' /proc/self/mountinfo | cut -d/ -f6")
     if not id:
       if 'microsoft' in uname().release: # wsl
         id = __exec__("powershell.exe -ExecutionPolicy bypass -command '(Get-CimInstance -Class Win32_ComputerSystemProduct).UUID'")
diff --git a/tests/__init__.py b/tests/__init__.py
index 2684511..d226c58 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -5,15 +5,15 @@
 
 class TestMachineId(unittest.TestCase):
   def test_hashed_id(self):
-    self.assertTrue(isinstance(machineid.hashed_id(), str))
-    self.assertTrue(len(machineid.hashed_id()) > 0)
-    self.assertTrue(machineid.hashed_id('foo') != machineid.hashed_id('bar'))
-    self.assertTrue(machineid.hashed_id('foo') != machineid.hashed_id())
+    self.assertIsInstance(machineid.hashed_id(), str)
+    self.assertGreater(len(machineid.hashed_id()), 0)
+    self.assertNotEqual(machineid.hashed_id('foo'), machineid.hashed_id('bar'))
+    self.assertNotEqual(machineid.hashed_id('foo'), machineid.hashed_id())
 
   def test_id(self):
-    self.assertTrue(isinstance(machineid.id(), str))
-    self.assertTrue(len(machineid.id()) > 0)
-    self.assertTrue(machineid.id() != machineid.hashed_id())
+    self.assertIsInstance(machineid.id(), str)
+    self.assertGreater(len(machineid.id()), 0)
+    self.assertNotEqual(machineid.id(), machineid.hashed_id())
 
 if __name__ == '__main__':
-  unittest.main()
\ No newline at end of file
+  unittest.main()