Bug 1341214 - Define GECKO in a single location. r=dustin

Instead of every file trying to get the top source directory having an
ad-hoc definition that gets wrong if the files gets moved around for
some reason, define it in a more central location.

taskcluster/taskgraph/util/docker.py

@@ -14,9 +14,9 @@ import tempfile
 from mozpack.archive import (
     create_tar_gz_from_files,
 )
+from .. import GECKO
 
 
-GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..', '..'))
 IMAGE_DIR = os.path.join(GECKO, 'taskcluster', 'docker')
 INDEX_PREFIX = 'docker.images.v2'
 ARTIFACT_URL = 'https://queue.taskcluster.net/v1/task/{}/artifacts/{}'