- Move the pnpm store, apk and .next caches into --mount=type=cache so
dependencies are shared across builds instead of duplicated in fresh
image layers (was the source of unbounded disk growth).
- Replace the deprecated --keep-storage prune flag in ci-deploy.sh with
the working --max-used-space=4g (buildx v0.37 renamed the flag). The
deprecated flag silently did nothing, so the BuildKit cache kept
growing unbounded (was 15.86GB); it is now capped at 4GB after every
deploy.