The ./hs Docker wrapper runs docker inside the Gitea runner container.
Since project paths inside the runner container don't exist on the Docker
host, the volume mount creates an empty /work directory, and Stack finds
no packages (error S-8506).
Fix: install Stack directly via get.haskellstack.org, use STACK_ROOT for
caching, and build with stack directly (no Docker-in-Docker).
Also add symlink for gitea-actions.sh convenience script.
