Import Gitea Arch registry key dynamically and configure pacman
with SigLevel = Required DatabaseOptional. Key ID is extracted
from the downloaded key file to avoid hardcoding.
Runner based on archlinux:base-devel with git, curl, makepkg.
Runs as non-root builder user so makepkg works natively without
permission workarounds. Registration data stored in /data volume.
The runner image is now built on archlinux:base-devel with git,
curl, makepkg and a non-root builder user baked in. This removes
the need for per-workflow pacman installs and enables host mode.
