fix act-runner: mount docker socket into job containers

Job containers need Docker socket to run docker/build-push-action
and docker/setup-buildx-action. Pass -v /var/run/docker.sock via
container.options in act_runner config.

act-runner/config.yaml

@@ -9,5 +9,8 @@ runner:
 container:
   network: "ci"
   privileged: true
-  options: ""
+  docker_host: unix:///var/run/docker.sock
+  valid_volumes:
+    - /var/run/docker.sock
+  options: "-v /var/run/docker.sock:/var/run/docker.sock"
   workdir_parent: ""