--- a/taskcluster/taskgraph/transforms/build.py
+++ b/taskcluster/taskgraph/transforms/build.py
@@ -24,19 +24,20 @@ def set_defaults(config, jobs):
         _, worker_os = worker_type_implementation(job['worker-type'])
         worker = job.setdefault('worker', {})
         if worker_os == "linux":
             worker.setdefault('docker-image', {'in-tree': 'desktop-build'})
             worker['chain-of-trust'] = True
             extra = job.setdefault('extra', {})
             extra.setdefault('chainOfTrust', {})
             extra['chainOfTrust'].setdefault('inputs', {})
-            extra['chainOfTrust']['inputs']['docker-image'] = {
-                "task-reference": "<docker-image>"
-            }
+            if 'in-tree' in worker['docker-image']:
+                extra['chainOfTrust']['inputs']['docker-image'] = {
+                    "task-reference": "<docker-image>"
+                }
         elif worker_os == "windows":
             worker.setdefault('env', {})
             worker['chain-of-trust'] = True
         elif worker_os == "macosx":
             worker.setdefault('env', {})
 
         yield job