diff --git a/decisionlib.py b/decisionlib.py
index bbe2ebc9dd1..55596e996ee 100644
--- a/decisionlib.py
+++ b/decisionlib.py
@@ -67,7 +67,7 @@ class DecisionTask:
                 raise
 
         return self.create_task(
-            task_name="docker image build task for image: " + self.image_name(dockerfile),
+            task_name="docker image build task for image: " + image_name(dockerfile),
             script="""
                 echo "$DOCKERFILE" | docker build -t taskcluster-built -
                 docker save taskcluster-built | lz4 > /%s
@@ -94,16 +94,6 @@ class DecisionTask:
             },
         )
 
-    def image_name(self, dockerfile):
-        basename = os.path.basename(dockerfile)
-        suffix = ".dockerfile"
-        if basename == "Dockerfile":
-            return os.path.basename(os.path.dirname(os.path.abspath(dockerfile)))
-        elif basename.endswith(suffix):
-            return basename[:-len(suffix)]
-        else:
-            return basename
-
     def create_task(self, *, task_name, script, max_run_time_minutes,
                     docker_image=None, dockerfile=None,  # One of these is required
                     artifacts=None, dependencies=None, env=None, cache=None, scopes=None,
@@ -195,5 +185,16 @@ class DecisionTask:
         return task_id
 
 
+def image_name(dockerfile):
+    basename = os.path.basename(dockerfile)
+    suffix = ".dockerfile"
+    if basename == "Dockerfile":
+        return os.path.basename(os.path.dirname(os.path.abspath(dockerfile)))
+    elif basename.endswith(suffix):
+        return basename[:-len(suffix)]
+    else:
+        return basename
+
+
 def deindent(string):
     return re.sub("\n +", " \n ", string)