Fix test-runner volume mount for DinD workspace

In Gitea runner pods, the repo checkout lives in a Docker volume, not
a host path. The `.:/work` mount in docker-compose.ci.yml resolved to
the Docker daemon's CWD (empty), causing "No such file or directory".

Resolve the actual Docker volume backing the CI workspace at runtime
and pass it via CI_WORKSPACE_VOLUME env var. Falls back to `.` for
local (non-DinD) usage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.ci.yml

@@ -31,7 +31,7 @@ services:
         condition: service_healthy
     working_dir: /work
     volumes:
-      - .:/work
+      - ${CI_WORKSPACE_VOLUME:-.}:/work
     environment:
       SERVER_URL: http://armature:80
       ADMIN_USER: admin