private static Optional<DockerImage> dockerImageToUse(ModelElement nodesElement, Optional<DockerImage> dockerImage) {
        String dockerImageFromElement = nodesElement.stringAttribute("docker-image");
        return dockerImageFromElement == null ? dockerImage : Optional.of(DockerImage.fromString(dockerImageFromElement));
    }
Returns the ID of the container element referencing nodesElement, if any
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/NodesSpecification.java