Added sockets extension

[mnocon]

It is needed by the new site-context package:

     - chrome-php/wrench[v1.5.0, ..., 1.6.x-dev] require ext-sockets * -> it is missing from your system. Install or enable PHP's sockets extension.
    - ibexa/site-context dev-IBX-7051-preview-btn-with-siteaccess requires chrome-php/chrome 1.10.x-dev -> satisfiable by chrome-php/chrome[1.10.x-dev].
    - chrome-php/chrome 1.10.x-dev requires chrome-php/wrench ^1.5 -> satisfiable by chrome-php/wrench[v1.5.0, 1.5.x-dev, 1.6.x-dev].
    - Root composer.json requires ibexa/site-context dev-IBX-7051-preview-btn-with-siteaccess as 4.6.x-dev -> satisfiable by ibexa/site-context[dev-IBX-7051-preview-btn-with-siteaccess].

Test failures are not related to this change (PHP 7.3 image was not changed)