Enable shared runner cache

The 1.9 release brings support for shared runner cache, cache that can be shared across different managers.

We should enable it.

Documentation is here: https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-cache-section.

I think that this is fine to enable shared cache across all runners, but it also makes sense to enable it between only gitlab-shared-X, docker-ci-X and shared-X with the help of Shared=true, Path=/gitlab-shared/.