ci: run dind under Sysbox, not privileged (rollback override kept)

dind was privileged: true, so a job that escaped into dind was root on
Veron 1, which is Grant's workstation. Under sysbox-runc (sysbox-ce 0.7.1,
installed 09-23 with no docker restart) dind root is an unprivileged host
uid. Smoke-tested standalone: nested containers, internet, a services-style
postgres on a private network and a python image all pass unprivileged.
Fresh volume dind-storage-sysbox; the old dind-storage stays for
docker-compose.privileged.yml, the one-command rollback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/runner/docker-compose.privileged.yml

@@ -0,0 +1,15 @@
+# ROLLBACK ONLY: the pre-Sysbox dind (privileged: true), kept one command away.
+# Use it if CI breaks under Sysbox:
+#
+#   cd /srv/windygit/src/deploy/runner
+#   sudo docker compose -f docker-compose.yml -f docker-compose.privileged.yml up -d dind
+#
+# (then restart the runners while idle). Compose merges `volumes` by container
+# path, so this puts back the old `dind-storage` volume with its image cache.
+# Going forward again: the same command without the second -f.
+services:
+  dind:
+    runtime: runc
+    privileged: true
+    volumes:
+      - dind-storage:/var/lib/docker