prevent polluting output by subprocesses

tools/run_tests/dockerjob.py

@@ -55,7 +55,7 @@ def docker_mapped_port(cid, port, timeout_seconds=15):
   while time.time() - started < timeout_seconds:
     try:
       output = subprocess.check_output('docker port %s %s' % (cid, port),
-                                       stderr=_DEVNULL
+                                       stderr=_DEVNULL,
                                        shell=True)
       return int(output.split(':', 2)[1])
     except subprocess.CalledProcessError as e:
@@ -85,7 +85,9 @@ def remove_image(image, skip_nonexistent=False, max_retries=10):
   if skip_nonexistent and not image_exists(image):
     return True
   for attempt in range(0, max_retries):
-    if subprocess.call(['docker','rmi', '-f', image]) == 0:
+    if subprocess.call(['docker','rmi', '-f', image],
+                       stdout=_DEVNULL,
+                       stderr=subprocess.STDOUT) == 0:
       return True
     time.sleep(2)
   print 'Failed to remove docker image %s' % image