Disable debug logs for image builds

Logs emitted when building via the tox environments can exceed the
maximum length supported by subunit [1], causing the following message:

Length too long: <length>

This change disables debug logging in these tox environments to reduce
the length of the output.

[1]
https://github.com/testing-cabal/subunit/blob/8cdd703980c314dddf95cc0556f67bc80358ddcd/python/subunit/v2.py#L208

Change-Id: Icb5793a8ea58edafe2f3a249eb1da7d4ae2e395f

tests/test_build.py

@@ -39,7 +39,7 @@ class BuildTest(object):
         if self.threads < 4:
             self.threads = 4
 
-        self.build_args = [__name__, "--debug", '--threads', str(self.threads)]
+        self.build_args = [__name__, '--threads', str(self.threads)]
 
     @testtools.skipUnless(os.environ.get('DOCKER_BUILD_TEST'),
                           'Skip the docker build test')