--- a/taskcluster/taskgraph/task/docker_image.py
+++ b/taskcluster/taskgraph/task/docker_image.py
@@ -59,38 +59,40 @@ class DockerImageTask(base.Task):
             'source': '{repo}file/{rev}/taskcluster/ci/docker-image/image.yml'
                       .format(repo=params['head_repository'], rev=params['head_rev']),
         }
 
         tasks = []
         templates = Templates(path)
         for image_name in config['images']:
             context_path = os.path.join('testing', 'docker', image_name)
-            context_hash = generate_context_hash(context_path)
 
             image_parameters = dict(parameters)
-            image_parameters['context_hash'] = context_hash
             image_parameters['context_path'] = context_path
             image_parameters['artifact_path'] = 'public/image.tar'
             image_parameters['image_name'] = image_name
 
             image_artifact_path = \
                 "public/decision_task/image_contexts/{}/context.tar.gz".format(image_name)
             if os.environ.get('TASK_ID'):
                 destination = os.path.join(
                     os.environ['HOME'],
                     "artifacts/decision_task/image_contexts/{}/context.tar.gz".format(image_name))
                 image_parameters['context_url'] = ARTIFACT_URL.format(
                     os.environ['TASK_ID'], image_artifact_path)
-                cls.create_context_tar(context_path, destination, image_name)
+                context_hash = cls.create_context_tar(context_path, destination,
+                                                      image_name)
             else:
                 # skip context generation since this isn't a decision task
                 # TODO: generate context tarballs using subdirectory clones in
                 # the image-building task so we don't have to worry about this.
                 image_parameters['context_url'] = 'file:///tmp/' + image_artifact_path
+                context_hash = generate_context_hash(context_path)
+
+            image_parameters['context_hash'] = context_hash
 
             image_task = templates.load('image.yml', image_parameters)
 
             attributes = {'image_name': image_name}
 
             # As an optimization, if the context hash exists for mozilla-central, that image
             # task ID will be used.  The reasoning behind this is that eventually everything ends
             # up on mozilla-central at some point if most tasks use this as a common image
@@ -128,22 +130,25 @@ class DockerImageTask(base.Task):
                 return True, existing_task['taskId']
             except urllib2.HTTPError:
                 pass
 
         return False, None
 
     @classmethod
     def create_context_tar(cls, context_dir, destination, image_name):
-        'Creates a tar file of a particular context directory.'
+        """Creates a tar file of a particular context directory.
+
+        Returns the SHA-256 hex digest of the created file.
+        """
         destination = os.path.abspath(destination)
         if not os.path.exists(os.path.dirname(destination)):
             os.makedirs(os.path.dirname(destination))
 
-        create_context_tar(context_dir, destination, image_name)
+        return create_context_tar(context_dir, destination, image_name)
 
     @classmethod
     def from_json(cls, task_dict):
         # Generating index_paths for optimization
         routes = task_dict['task']['routes']
         index_paths = []
         for route in routes:
             index_path_regex = re.compile(INDEX_REGEX)