fix(dind): double-fork in s6-notifyoncheck to eliminate zombie processes (#1364)

In `scripts/s6/docker/run`, `s6-notifyoncheck` runs without the `-d` flag:

```bash
exec s6-notifyoncheck -n 0 dockerd-entrypoint.sh
```

Without `-d`, `s6-notifyoncheck` performs a single fork to execute the background checking process. The parent immediately replaces itself via `exec` with `dockerd-entrypoint.sh` (which calls `exec rootlesskit` on `dind-rootless`).

Because only a single fork occurred, the background checking child remains parented to `rootlesskit`. When the check passes and the process terminates:
1. `rootlesskit` did not spawn this process and never reaps it via `waitpid()`.
2. Under standard Linux process semantics, `s6-svscan` (PID 1) cannot adopt or reap the child because its parent (`rootlesskit`) is still running.
3. The terminated process remains as a permanent zombie (`[s6-notifyoncheck] <defunct>`) for the container's entire lifespan.

Adding `-d` instructs `s6-notifyoncheck` to double-fork, detaching the background check so it is immediately adopted by `s6-svscan` (PID 1) and cleanly reaped upon exit.

---------

Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1364
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: adamaru <232090+adamaru@noreply.gitea.com>

scripts/s6/docker/run

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-exec s6-notifyoncheck -n 0 dockerd-entrypoint.sh
+exec s6-notifyoncheck -d -n 0 dockerd-entrypoint.sh