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]>

act/container/docker_stub.go

@@ -18,7 +18,7 @@ import (
 	"github.com/moby/moby/api/types/system"
 )
 
-func IsolatedNetwork(context.Context, netip.Addr, string) (string, error) {
+func IsolatedCacheContainer(context.Context, netip.Addr, string) (string, error) {
 	return "", nil
 }
 
@@ -91,7 +91,19 @@ func NewDockerNetworkRemoveExecutor(name string) common.Executor {
 	}
 }
 
-func RemoveOrphanNetworks(ctx context.Context, runnerUUID string, createdBefore time.Time) error {
+func NewDockerNetworkConnectExecutor(_, _ string) common.Executor {
+	return func(_ context.Context) error {
+		return nil
+	}
+}
+
+func NewDockerNetworkDisconnectExecutor(_, _ string) common.Executor {
+	return func(_ context.Context) error {
+		return nil
+	}
+}
+
+func RemoveOrphanNetworks(_ context.Context, _, _ string, _ time.Time) error {
 	return nil
 }