fix: dind image readiness, exit codes and shutdown order (#1265)

Fixes https://gitea.com/gitea/runner/issues/1263

- Wait for dockerd without a check limit. s6 readiness gave up after 7 checks, so the runner never started on slow hosts, also seen in https://gitea.com/gitea/runner/issues/712, https://gitea.com/gitea/runner/issues/652 and https://gitea.com/gitea/runner/issues/758.
- Let the runner run the docker service's readiness check itself instead of reading s6 status, which can be missing at startup or stale after an unclean stop.
- Exit the container with the first failing service's exit code instead of always 0, so `restart: on-failure` works. A requested stop still exits 0.
- Stop the runner before dockerd, so running jobs can finish within `shutdown_timeout`. The stop grace period has to cover that drain.
- Check the supervision exit codes in `make test-dind`.

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1265
Reviewed-by: bircni <[email protected]>

scripts/s6/.s6-svscan/exitcode-path

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+echo "/etc/s6/.s6-svscan/exitcode.$(cat /proc/sys/kernel/random/boot_id).$(cut -d ' ' -f 22 /proc/1/stat)" # unique per container start, so a killed run's record is ignored