fix: wait for docker supervise dir before s6-svwait (#851)

`s6-svscan` starts services in parallel, so `act_runner/run` could invoke `s6-svwait` before s6 had created the docker service's `supervise/` directory, failing with `s6-svwait: fatal: unable to s6_svstatus_read: No such file or directory`. Poll for the directory before waiting.

Fixes #760

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/runner/pulls/851
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

scripts/s6/act_runner/run

@@ -1,5 +1,9 @@
 #!/usr/bin/env bash
 
+while [ ! -d /etc/s6/docker/supervise ]; do
+    sleep 0.1
+done
+
 s6-svwait -U /etc/s6/docker
 
 exec run.sh