fix: use container skopeo (#110)

the `os_version` is defaulting to the `image_tag` inside containers and
causing our template to use latest tag

---------

Co-authored-by: Gerald Pinder <gmpinder@gmail.com>

template/src/lib.rs

@@ -21,6 +21,9 @@ pub struct ContainerFileTemplate<'a> {
 
     #[builder(default)]
     export_script: ExportsTemplate,
+
+    #[builder(setter(into))]
+    os_version: Cow<'a, str>,
 }
 
 #[derive(Debug, Clone, Default, Template)]