feat!: serve the actions cache from the runner (#1229)

Make cache requests go through the runner and remove outbound traffic from the cache server. The runner forwards them to `cache.external_server` when one is set, and attaches itself to each job network so jobs can reach it.

Benefits:

1. Jobs need no http proxy rules for the cache server, which `@actions/http-client` mishandles for CIDR ranges in `no_proxy` (https://github.com/actions/toolkit/pull/2495).
2. Only runners reach a shared cache server, which needs no route to Gitea.
3. A runner container sharing the host Docker socket now caches, where jobs previously could not reach it.

Breaks:

1. Jobs that reach `external_server` but not their runner, such as a remote `DOCKER_HOST`. The built-in cache has always needed that reachability, so both modes now agree. Set `cache.host` and `cache.port` to an address those jobs reach.
2. Jobs on `container.network: bridge`, where Docker rejects the alias, so the runner cannot attach and their cache fails as it does today. Leave `container.network` empty, or name a user-defined network.

*Written by Claude.*

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1229
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

README.md

@@ -312,7 +312,7 @@ Use `uses: builtin:checkout` to check out a repository without downloading `acti
 
 #### Caching (`actions/cache`)
 
-Each runner starts its own cache server, so runners do not share cached entries. When the runner itself runs in Docker, set `cache.host` to an address job containers can reach and `cache.port` to a fixed published port, or put jobs on a shared `container.network`.
+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.
 
 **Sharing a cache between runners**
 
@@ -337,7 +337,7 @@ cache:
   external_secret: "<secret>"
 ```
 
-Jobs connect to `external_server` too, so point it at the reverse proxy if one fronts the server. `--dir`, `--host` and `--port` override the matching `cache` keys. Eviction settings take effect on the cache server, not on the runners.
+`--dir`, `--host` and `--port` override the matching `cache` keys. Eviction settings take effect on the cache server, not on the runners.
 
 Runners can also share one `cache.dir` on a file system with working file locks, at the cost of slower cache requests.