builder/docker: don't attempt to read artifact if cancelled

builder/docker/builder.go

@@ -77,8 +77,13 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 		return nil, rawErr.(error)
 	}
 
-	var artifact packer.Artifact
+	// If it was cancelled, then just return
+	if _, ok := state.GetOk(multistep.StateCancelled); ok {
+		return nil, nil
+	}
+
 	// No errors, must've worked
+	var artifact packer.Artifact
 	if b.config.Commit {
 		artifact = &ImportArtifact{
 			IdValue:        state.Get("image_id").(string),
@@ -88,6 +93,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	} else {
 		artifact = &ExportArtifact{path: b.config.ExportPath}
 	}
+
 	return artifact, nil
 }