The role had drifted from what runs on asxpio-services: capacity 4,
a bridge network per workflow so service containers start, the Docker
socket automounted into jobs, and a nix-latest label backed by a
persistent nix-store volume.
That volume caused a CI outage on 2026-10-18. nixos/nix:latest moved on,
and since the image's /bin/sh and /etc/nix/nix.conf link into /nix/store,
the old volume left them dangling. Pin the image through
forgejo_runner_nix_version and name the volume after it, so a bump is
deliberate and always gets a store seeded from its own image.
Assisted-by: Claude:opus-5-5
