armui.py code cleanup

Properly handle file checks, removing resource leak, closing file once read.

Author: microtechno9000

arm/runui.py

@@ -44,10 +44,15 @@ def is_docker():
     returns: Boolean
     """
     path = '/proc/self/cgroup'
-    return (
-            os.path.exists('/.dockerenv') or
-            os.path.isfile(path) and any('docker' in line for line in open(path))
-    )
+
+    if os.path.exists('/.dockerenv'):
+        return True
+
+    if os.path.isfile(path):
+        with open(path) as f:
+            return any('docker' in line for line in f)
+
+    return False
 
 
 def get_host():