private static Map<String, String> getEnvironmentVariables(Element environmentVariables) {
        var map = new LinkedHashMap<String, String>();
        if (environmentVariables != null) {
            for (Element var: XML.getChildren(environmentVariables)) {
                var name = new com.yahoo.text.Identifier(var.getNodeName());
                map.put(name.toString(), var.getTextContent());
            }
        }
        return map;
    }
Add nodes to cluster according to the given containerElement.

Note: DO NOT change allocation behaviour to allow version X and Y of the config-model to allocate a different set
of nodes. Such changes must be guarded by a common condition (e.g. feature flag) so the behaviour can be changed
simultaneously for all active config models.
config-model/src/main/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilder.java