feat(cache): add shared S3-compatible storage (#1244)

Store the cache used by `actions/cache` in an S3-compatible bucket such as AWS S3 or RustFS, with no `cache-server`. Runners with the same settings use the same cache.

A cache saved on one runner can be restored on the others within 30 seconds. The runner talks to the bucket over the S3 REST API with SigV4, without an SDK. Lifecycle rules on the bucket expire old caches.

*Description rewritten by Claude.*

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1244
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

internal/pkg/config/config.example.yaml

@@ -166,6 +166,19 @@ cache:
   # Surrounding whitespace is trimmed, so a trailing newline in the file is fine.
   # Setting both external_secret and external_secret_file is an error.
   #external_secret_file: ""
+  # Store the cache in an S3-compatible bucket such as AWS S3, MinIO or RustFS. Runners with
+  # the same settings use the same cache. Cannot be combined with external_server. Add lifecycle
+  # rules to the bucket that expire old caches and abort incomplete multipart uploads.
+  # region defaults to us-east-1, and path_style to false for AWS and true otherwise.
+  # access_key and secret_key can also be set directly instead of read from files.
+  #s3:
+  #  endpoint: "http://minio:9000"
+  #  region: "us-east-1"
+  #  bucket: "runner-cache"
+  #  prefix: "gitea-runner-cache"
+  #  path_style: true
+  #  access_key_file: "/run/secrets/cache-s3-access-key"
+  #  secret_key_file: "/run/secrets/cache-s3-secret-key"
   # When true, reuse a cached action instead of fetching from the remote on every job.
   # A moved tag (e.g. a re-tagged "v6") or an updated branch stays at the cached commit
   # until its cache entry expires or is manually removed.
@@ -176,8 +189,8 @@ cache:
   # advertised. A bundle that does not match is left alone. With v2, artifact uploads go via the
   # cache server.
   #v2: true
-  # How the cache server discards entries, ignored when external_server is set since that
-  # server applies its own. Leave a setting out for its default; 0s or 0 turns the three
+  # How the cache server discards entries, ignored when external_server or s3 is set since
+  # those apply their own. Leave a setting out for its default; 0s or 0 turns the three
   # limits off. Sizes accept 10GB, 512mb, 1TiB or a plain byte count, binary either way.
   # Whatever these allow, the cache still sheds entries to keep free space on its volume
   # above health_check.min_free_disk_space_mb when health checks are enabled.