Move tests docker configuration

Because we want to unify tests environment in the premium plugin. This step should allow us to reuse it easier.
[PREMIUM-230]

tests_env/docker/install-extensions.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -e
+
+# Install PHP extensions required by MailPoet
+docker-php-ext-install pdo_mysql
+
+# Execute the original command
+exec "$@"