diff --git a/taskcluster/docs/attributes.rst b/taskcluster/docs/attributes.rst
index 8fc75ec5fe4a..ee69bcf33b0c 100644
--- a/taskcluster/docs/attributes.rst
+++ b/taskcluster/docs/attributes.rst
@@ -255,3 +255,14 @@ cron
 ====
 Indicates that a task is meant to be run via cron tasks, and should not be run
 on push.
+
+cache_digest
+============
+Some tasks generate artifacts that are cached between pushes. This is the unique string used
+to identify the current version of the artifacts. See :py:mod:`taskgraph.util.cached_task`.
+
+cache_type
+==========
+Some tasks generate artifacts that are cached between pushes. This is the type of cache that is
+used for the this task. See :py:mod:`taskgraph.util.cached_task`.
+
diff --git a/taskcluster/taskgraph/transforms/docker_image.py b/taskcluster/taskgraph/transforms/docker_image.py
index bff7002f0c00..681849dea3e1 100644
--- a/taskcluster/taskgraph/transforms/docker_image.py
+++ b/taskcluster/taskgraph/transforms/docker_image.py
@@ -91,15 +91,10 @@ def fill_template(config, tasks):
         if task.kind != 'packages':
             continue
         name = task.label.replace('packages-', '')
-        for route in task.task.get('routes', []):
-            if route.startswith('index.') and '.hash.' in route:
-                # Only keep the hash part of the route.
-                h = route.rsplit('.', 1)[1]
-                assert DIGEST_RE.match(h)
-                available_packages[name] = h
-                break
+        available_packages[name] = task.attributes['cache_digest']
 
     context_hashes = {}
+    image_digests = {}
 
     for task in order_image_tasks(config, tasks):
         image_name = task.pop('name')
@@ -130,6 +125,9 @@ def fill_template(config, tasks):
         digest_data = [context_hash]
         context_hashes[image_name] = context_hash
 
+        if parent:
+            digest_data += [image_digests[parent]]
+
         description = 'Build the docker image {} for use by dependent tasks'.format(
             image_name)
 
@@ -247,4 +245,6 @@ def fill_template(config, tasks):
             **kwargs
         )
 
+        image_digests[image_name] = taskdesc['attributes']['cache_digest']
+
         yield taskdesc
diff --git a/taskcluster/taskgraph/util/cached_tasks.py b/taskcluster/taskgraph/util/cached_tasks.py
index 3c9c54055b3f..d29d2c6c77ac 100644
--- a/taskcluster/taskgraph/util/cached_tasks.py
+++ b/taskcluster/taskgraph/util/cached_tasks.py
@@ -68,6 +68,9 @@ def add_optimization(config, taskdesc, cache_type, cache_name, digest=None, dige
         for route in EXTRA_CACHE_INDEXES
     ])
 
+    taskdesc['attributes']['cache_digest'] = digest
+    taskdesc['attributes']['cache_type'] = cache_type
+
 
 def cached_index_path(level, trust_domain, cache_type, cache_name, digest=None, digest_data=None):
     """