Coolify pulls from GitHub and runs docker compose build — no guaranteed
env vars like SOURCE_COMMIT. Previous approach relied on ARG/ENV
passthrough that silently defaulted to "dev".
Fix: install git in the builder stage, COPY .git into context, run
git describe --tags --always to derive the version, write it to
/app/server/.bf-version. version.ts reads this file as a fallback
between env vars and the "dev" literal.
Chain: BF_SERVER_VERSION env → BF_BUILD_VERSION env → .bf-version file
→ COOLIFY_GIT_COMMIT env → SOURCE_COMMIT env → "dev".
Also: fix .gitignore for rauc-signing/ (was under wrong path).
