fix act-runner: attach job containers to ci network

Job containers spawned by act_runner couldn't resolve 'gitea'
hostname — they were on isolated per-job networks. Setting
GITEA_RUNNER_CONTAINER_NETWORK=ci puts them on the same network
as gitea, enabling git fetch during checkout.

compose.yml

@@ -227,6 +227,7 @@ services:
       - GITEA_RUNNER_REGISTRATION_TOKEN=${ACT_RUNNER_TOKEN}
       - GITEA_RUNNER_NAME=homelab-runner
       - GITEA_RUNNER_LABELS=ubuntu-latest:docker://catthehacker/ubuntu:act-latest
+      - GITEA_RUNNER_CONTAINER_NETWORK=ci
     volumes:
       - ${DOCKER_SOCK:-/run/user/1000/docker.sock}:/var/run/docker.sock
       - act_runner_data:/data