Document howto run Salsa-CI locally

It is possible to run gitlab-runner locally using gitlab-runner exec.

I have never tried this, but I decided to file an issue about it so we can record people's experiences if/when somebody tries it out to document the exact commands and preparations needed to specifically run the Salsa-CI pipeline locally.

The motivation is that some builds are quite heavy and instead of testing the full pipeline by pushing to Salsa (and consuming Salsa computing resources) some developer might want to re-run the pipeline locally and many only partially to check how certain small local modifications changed the end result.