docs(templates): add a section about the registry field

docs/source/templates.rst

@@ -56,6 +56,13 @@ The Docker image associated to the template. The image tag **must** be included.
 
 This field is **mandatory**.
 
+``registry``
+---------
+
+The registry where the Docker image is stored. If not specified, Portainer will use the Dockerhub as the default registry.
+
+This field is **optional**.
+
 ``env``
 -------