Add .gitlab-ci.yml file for the 8.x-1.x branch

The pipeline ran with it with no issues, but there's some PHPCS/ESLINT/.. issues that I'll address in another issue: https://git.drupalcode.org/project/sshop/-/pipelines/492369

I'll merge this to the 8.x-1.x branch, but I'll leave it to "Needs Review" for now, until I'm sure this theme can be installed on both D10/D11.

I was able to make it work on both D10/D11, I'll post a tagged release soon, there's a lot of issues to be fixed in this theme, for now I'll mark this as "Fixed", if anyone has any concerns with the gitlab configuration, please let me know and I'll re-open the task.