The recommended way to deploy Drupal 8 on Platform.sh is to use Composer.
Composer is a PHP package manager that controls which versions of your dependencies are pulled into your project on every build.
With a Composer-installed Drupal project, it is possible to update your entire application with a single command and commit.
Checkout a new branch on your local project and run
composer update to update your dependencies.
Push to Platform.sh and then activate the environment to deploy the updated application.
Once the updates have passed all of your tests, you can merge and deploy the exact same updated application to production in seconds.