gitlab-ci: Only run the pipeline for changes to files affecting it

Merged Michel Dänzer requested to merge daenzer/mesa:gitlab-ci-only-changes.diff into master

E.g. documentation-only changes cannot affect the outcome of the pipeline, so don't waste resources on running it.

The thing we need to be careful about here is that the container stage jobs must always run if any later stage jobs using the corresponding docker images run. We're currently using the same .ci-run-policy template for all jobs, so this is trivially true.

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