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/docker.py

@@ -18,8 +18,8 @@ from subprocess import Popen, PIPE
 from io import BytesIO
 
 from taskgraph.util import docker
+from . import GECKO
 
-GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..'))
 INDEX_URL = 'https://index.taskcluster.net/v1/task/' + docker.INDEX_PREFIX + '.{}.{}.hash.{}'
 ARTIFACT_URL = 'https://queue.taskcluster.net/v1/task/{}/artifacts/{}'