Support Deployer 8.x - missing sockets ext.

[Muffinman]

Hi,

The images currently do not build with the sockets extension, which means they can not currently run Deployer on the 8.x branch, as this is a strict requirement.

Please could the sockets extension be added?

$ vendor/bin/dep deploy staging -v
deploy:info
In Server.php line 51:
                                                  
  [Deployer\Exception\Exception]                  
  Required PHP extension "sockets" is not loaded  
                                                  
Exception trace:
  at /builds/websites/my-project/vendor/deployer/deployer/src/Executor/Server.php:51
 Deployer\Executor\Server::checkRequiredExtensionsExists() at /builds/websites/my-project/vendor/deployer/deployer/src/Executor/Server.php:42
 Deployer\Executor\Server->__construct() at /builds/websites/my-project/vendor/deployer/deployer/src/Executor/Master.php:166
 Deployer\Executor\Master->runTask() at /builds/websites/my-project/vendor/deployer/deployer/src/Executor/Master.php:110
 Deployer\Executor\Master->run() at /builds/websites/my-project/vendor/deployer/deployer/src/Command/MainCommand.php:135
 Deployer\Command\MainCommand->execute() at /builds/websites/my-project/vendor/symfony/console/Command/Command.php:318
 Symfony\Component\Console\Command\Command->run() at /builds/websites/my-project/vendor/symfony/console/Application.php:1092
 Symfony\Component\Console\Application->doRunCommand() at /builds/websites/my-project/vendor/symfony/console/Application.php:359
 Symfony\Component\Console\Application->doRun() at /builds/websites/my-project/vendor/symfony/console/Application.php:194
 Symfony\Component\Console\Application->run() at /builds/websites/my-project/vendor/deployer/deployer/src/Deployer.php:289
 Deployer\Deployer::run() at /builds/websites/my-project/vendor/deployer/deployer/bin/dep:103
 include() at /builds/websites/my-project/vendor/bin/dep:119
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