private static Optional<String> containerIdOf(ModelElement nodesElement) {
        var element = nodesElement.getXml();
        var container = findParentByTag("container", element);
        return container.map(el -> el.getAttribute("id"));
    }
Returns the ID of the parent container element of nodesElement, if any
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/NodesSpecification.java