examples: improve DIND rootless network performance (#786)

## Summary
- Add `DOCKERD_ROOTLESS_ROOTLESSKIT_NET=slirp4netns` and `DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=65520` to the DIND docker-compose example
- The `docker:dind-rootless` base image defaults to vpnkit as the network driver, which has substantially lower throughput than slirp4netns

## The problem

I noticed that pulling containers as well as downloading data within the container when running act_runner as DIND was very slow (see Ookla speedtest results in the following). While analysing the issue, I found that this was caused by the usage of vpnkit.

The `docker:dind-rootless` base image defaults to vpnkit as the network driver. slirp4netns was [added as an opt-in option](https://github.com/docker-library/docker/pull/543) and must be explicitly enabled via `DOCKERD_ROOTLESS_ROOTLESSKIT_NET=slirp4netns`.

This means anyone following the current DIND example gets vpnkit, which has significantly lower network throughput. This affects **all** network operations in the container — image pulls, package installs, and CI tasks.

Per the [rootlesskit iperf3 benchmarks](https://github.com/rootless-containers/rootlesskit/blob/master/docs/network.md):

| Driver | MTU 1500 | MTU 65520 |
|--------|----------|-----------|
| **vpnkit** | 0.60 Gbps | not supported |
| **slirp4netns** | 1.06 Gbps | 7.55 Gbps |

## Real-world benchmark results (Ookla speedtest, same server)

| | Download | Upload |
|---|---|---|
| **Default (vpnkit)** | ~130 Mbps | ~126 Mbps |
| **slirp4netns + MTU 65520** | ~958 Mbps | ~462 Mbps |

## References
- [docker-library/docker#543](https://github.com/docker-library/docker/pull/543) — added slirp4netns to dind-rootless as opt-in (vpnkit remains default)
- [rootlesskit network docs](https://github.com/rootless-containers/rootlesskit/blob/master/docs/network.md) — iperf3 benchmarks

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/786
Reviewed-by: silverwind <silverwind@noreply.gitea.com>
Co-authored-by: stecklars <stecklars@noreply.gitea.com>
Co-committed-by: stecklars <stecklars@noreply.gitea.com>

examples/docker-compose/README.md

@@ -53,6 +53,9 @@
     environment:
       - GITEA_INSTANCE_URL=<instance url>
       - DOCKER_HOST=unix:///var/run/user/1000/docker.sock
+      # Use slirp4netns instead of vpnkit for significantly better network throughput.
+      - DOCKERD_ROOTLESS_ROOTLESSKIT_NET=slirp4netns
+      - DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=65520
       # When using Docker Secrets, it's also possible to use
       # GITEA_RUNNER_REGISTRATION_TOKEN_FILE to pass the location.
       # The env var takes precedence.