--- a/taskcluster/taskgraph/util/docker.py
+++ b/taskcluster/taskgraph/util/docker.py
@@ -68,32 +68,54 @@ def create_context_tar(topsrcdir, contex
 
     A directory ``context_dir`` containing a Dockerfile will be assembled into
     a gzipped tar file at ``out_path``. Files inside the archive will be
     prefixed by directory ``prefix``.
 
     We also scan the source Dockerfile for special syntax that influences
     context generation.
 
-    If a line in the Dockerfile has the form ``# %include <path>``,
+    If a line in the ``Dockerfile`` has the form ``# %include <path>``,
     the relative path specified on that line will be matched against
     files in the source repository and added to the context under the
     path ``topsrcdir/``. If an entry is a directory, we add all files
     under that directory.
 
     Returns the SHA-256 hex digest of the created archive.
     """
+    if os.path.exists(os.path.join(context_dir, 'Dockerfile')):
+        archive_files = _archive_files_from_dockerfile(topsrcdir, context_dir)
+    else:
+        raise Exception('do not know how to build docker image from %s; must '
+                        'have Dockerfile' % context_dir)
+
+    archive_files = {os.path.join(prefix, k): v
+                     for k, v in archive_files.items()}
+
+    with open(out_path, 'wb') as fh:
+        create_tar_gz_from_files(fh, archive_files, '%s.tar.gz' % prefix)
+
+    h = 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()
+
+
+def _archive_files_from_dockerfile(topsrcdir, context_dir):
     archive_files = {}
 
     for root, dirs, files in os.walk(context_dir):
         for f in files:
             source_path = os.path.join(root, f)
             rel = source_path[len(context_dir) + 1:]
-            archive_path = os.path.join(prefix, rel)
-            archive_files[archive_path] = source_path
+            archive_files[rel] = source_path
 
     # Parse Dockerfile for special syntax of extra files to include.
     with open(os.path.join(context_dir, 'Dockerfile'), 'rb') as fh:
         for line in fh:
             line = line.rstrip()
             if not line.startswith('# %include'):
                 continue
 
@@ -108,33 +130,25 @@ def create_context_tar(topsrcdir, contex
 
             if not os.path.exists(fs_path):
                 raise Exception('extra include path does not exist: %s' % p)
 
             if os.path.isdir(fs_path):
                 for root, dirs, files in os.walk(fs_path):
                     for f in files:
                         source_path = os.path.join(root, f)
-                        archive_path = os.path.join(prefix, 'topsrcdir', p, f)
+                        archive_path = os.path.join('topsrcdir', p, f)
                         archive_files[archive_path] = source_path
             else:
-                archive_path = os.path.join(prefix, 'topsrcdir', p)
+                archive_path = os.path.join('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)
+    return archive_files
 
-    h = 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()
+
 
 
 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.