CI: create a registry.gitlab.com/scilab/scilab/linux-runner for toolboxes

Toolboxes need a CI to deliver/tests features easily. This commit introduces tagged docker images named registry.gitlab.com/scilab/scilab/linux-runner to execute toolbox pipelines using the same tagged Scilab version.

This base image can also be customized for multiple usages, including Binder images, Toolbox-specific images, etc...