enhance(cache): scope cache entries by ref like GitHub (#1361)

Cache entries were only scoped by repository, so an entry a pull request saved could be restored on the default branch, for example through a `restore-keys` prefix.

Entries now carry the ref of the job that saved them. As on GitHub ([Restrictions for accessing a cache](https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching#restrictions-for-accessing-a-cache)), a job restores entries of its own ref first, then of its pull request's base branch, then of the default branch. Pull request events are scoped by their pull request number.

Entries saved before this change are no longer restored and expire through `cache.retention`, so caches start cold once after the upgrade. Runners sharing a cache, and an external `cache-server`, should be upgraded together.

Co-authored-by: silverwind <[email protected]>
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1361
Reviewed-by: silverwind <[email protected]>
Co-authored-by: JorgeDLMV <[email protected]>

README.md

@@ -303,6 +303,8 @@ Use `uses: builtin:checkout` to check out a repository without downloading `acti
 
 Each runner starts its own cache server, so runners do not share cached entries. A runner in a container on the jobs' Docker daemon attaches itself to each job network, so jobs reach its cache without extra setup.
 
+As on GitHub, a job restores caches saved for its own ref, its pull request's base branch and the default branch, so a cache saved by a pull request is only restored by that pull request.
+
 **Sharing a cache between runners**
 
 Run one `gitea-runner cache-server` and point every runner at it, using the same secret everywhere, for example from `openssl rand -hex 32`: