--- a/taskcluster/taskgraph/util/docker.py
+++ b/taskcluster/taskgraph/util/docker.py
@@ -5,23 +5,24 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import hashlib
 import os
 import shutil
 import subprocess
 import tarfile
 import tempfile
+import json
 
 from mozbuild.util import memoize
 from mozpack.archive import (
     create_tar_gz_from_files,
 )
 from .. import GECKO
-
+from .hash import hash_path
 
 IMAGE_DIR = os.path.join(GECKO, 'taskcluster', 'docker')
 INDEX_PREFIX = 'docker.images.v2'
 
 
 def docker_image(name, by_tag=False):
     '''
         Resolve in-tree prebuilt docker image to ``<registry>/<repository>@sha256:<digest>``,
@@ -117,24 +118,35 @@ def create_context_tar(topsrcdir, contex
                         archive_files[archive_path] = source_path
             else:
                 archive_path = os.path.join(prefix, 'topsrcdir', p)
                 archive_files[archive_path] = fs_path
 
     with open(out_path, 'wb') as fh:
         create_tar_gz_from_files(fh, archive_files, '%s.tar.gz' % prefix)
 
-    h = hashlib.sha256()
+    # Compute hash of context tarball
+    ctx_hash = hashlib.sha256()
     with open(out_path, 'rb') as fh:
         while True:
             data = fh.read(32768)
             if not data:
                 break
-            h.update(data)
-    return h.hexdigest()
+            ctx_hash.update(data)
+    # Construct image hash as combination of other hashes
+    return hashlib.sha256(json.dumps({
+        "version": 1,  # version number for manually bumping
+        "context": ctx_hash.hexdigest(),  # hash of context tarball
+        "code": hash_path(__file__),  # hash of file that computes hashes
+        # hash of file that constructs the image-task
+        "task": hash_path(os.path.abspath(os.path.join(
+            os.path.dirname(__file__),
+            os.path.join(os.path.pardir, 'transforms', 'docker_image.py'),
+        ))),
+    }, sort_keys=True)).hexdigest()
 
 
 def build_from_context(docker_bin, context_path, prefix, tag=None):
     """Build a Docker image from a context archive.
 
     Given the path to a `docker` binary, a image build tar.gz (produced with
     ``create_context_tar()``, a prefix in that context containing files, and
     an optional ``tag`` for the produced image, build that Docker image.