fix(cache): expose embedded cache to job containers via host

act_runner's defaults autodetect its own container IP as the cache
host, which per-job networks can't route to — every actions/cache
restore and save hit a 5-minute ETIMEDOUT. Add a config.yaml that
pins the cache port and advertises host.docker.internal, publish
that port on the host, and inject --add-host=host-gateway into
job containers so the hostname resolves inside isolated networks.

config.yaml

@@ -0,0 +1,8 @@
+cache:
+  enabled: true
+  dir: "/data/cache"
+  host: "host.docker.internal"
+  port: 42951
+
+container:
+  options: "--add-host=host.docker.internal:host-gateway"