Steps to reproduce the issue

Log into the administrator backend
go to Global Configuration / System
In Cache Settings

set Redis Server Host to /var/run/redis/redis.sock
set Redis Server Port to 0
set Redis xxxxx to match your redis server

Expected result

Accept the port number 0
Save the configuration.
Print the Message
Configuration successfully saved.

Actual result

Error
Invalid field: Redis Server Port

The configuration System cannot be saved and joomla cannot connect to the redis server via socket.

System information (as much as possible)

FreeBSD 11.0-RELEASE-p9
Apache/2.4.25
PHP 7.1.4 fpm-cgi
MySQL 5.7.18-log
Joomla! 3.7.0 Stable

Additional comments

The administrator backend refuse to validate Redis Server Port to 0 or -1 but it looks like this is needed for joomla to connect to a redis server socket.
So I did hardcode the redis port to 0 in libraries/joomla/cache/storage/redis.php

 $server = array(
            'host' => $app->get('redis_server_host', 'localhost'),
            //'port' => $app->get('redis_server_port', 6379),
            'port' => 0,
            'auth' => $app->get('redis_server_auth', null),
            'db'   => (int) $app->get('redis_server_db', null),
        );

and now joomla is able to connect to redis socket and I am able to save the change made in the Global Configuration.