Bug 1288567 - Pass topsrcdir into create_context_tar; r=dustin

Relying on global variables like GECKO is a bit dangerous. To facilitate
testing of archive generation in subsequent commits, let's pass an
path into this function.

The argument is currently unused.

MozReview-Commit-ID: Et1UYraflDP

--HG--
extra : rebase_source : 012095bc2450c72467f3f65f71bcb4cf6efcde66

taskcluster/taskgraph/task/docker_image.py

@@ -143,7 +143,7 @@ class DockerImageTask(base.Task):
         if not os.path.exists(os.path.dirname(destination)):
             os.makedirs(os.path.dirname(destination))
 
-        return create_context_tar(context_dir, destination, image_name)
+        return create_context_tar(GECKO, context_dir, destination, image_name)
 
     @classmethod
     def from_json(cls, task_dict):

taskcluster/taskgraph/util/docker.py

@@ -60,7 +60,7 @@ def generate_context_hash(image_path):
     return context_hash.hexdigest()
 
 
-def create_context_tar(context_dir, out_path, prefix):
+def create_context_tar(topsrcdir, context_dir, out_path, prefix):
     """Create a context tarball.
 
     A directory ``context_dir`` containing a Dockerfile will be assembled into