chore(containers): ignore stderr of `gcloud container images list-tag…

…s` (#22) (#24)

A manual cherry-pick of #20

Co-authored-by: Michael Plump <plumpy@gmail.com>

dev/buildtool/container_commands.py

@@ -18,6 +18,7 @@
 import logging
 import os
 import shutil
+import subprocess
 
 from buildtool import (
   SPINNAKER_HALYARD_REPOSITORY_NAME,
@@ -69,7 +70,7 @@ def __gcb_image_exists(self, image_name, version):
                options.docker_registry + '/' + image_name,
                '--filter="%s"' % version,
                '--format=json']
-    got = check_subprocess(' '.join(command))
+    got = check_subprocess(' '.join(command), stderr=subprocess.PIPE)
     if got.strip() != '[]':
       return True
     return False