Global before_ and after_ script defined by admin

Description

To setup CI environment inside of a Docker container we need to run few .sh scripts. We add to each of our .gitlab-ci.yml: before_script and after_script that always looks the same. Maybe we could introduce global script definitions? Maybe in config.toml or somewhere else? It would be very useful to setup and tear down environment before tasks that regular developer can achieve inside .gitlab-ci.yml.

Proposal

Add global before_script and after_script. Maybe inside config.toml or elsewhere inside /etc/gitlab-runner.