private Integer getPortFromProjectOrSystemProperty(String var) {
        String sysProp = System.getProperty(var);
        if (sysProp != null) {
            return getAsIntOrNull(sysProp);
        }
        if (projProperties.containsKey(var)) {
            return getAsIntOrNull(projProperties.getProperty(var));
        }
        return null;
    }
Return the content of the mapping as an array with all specifications as given

@return port mappings as JSON array or null if no mappings exist
src/main/java/io/fabric8/maven/docker/access/PortMapping.java