fix(ci): bound Docker build cache with BuildKit cache mounts

- 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.

scripts/ci-deploy.sh

@@ -176,5 +176,5 @@ while IFS= read -r tag; do
     if [ -n "$tagged_image" ] && [ "$tagged_image" != "$previous_image" ]; then docker image rm "$tag" || true; fi
   fi
 done < <(docker image ls --format '{{.Repository}}:{{.Tag}}' epicnext-cms)
-docker builder prune -af --filter "until=72h" --keep-storage=2g || true
+docker builder prune -af --filter "until=72h" --max-used-space=4g || true
 docker image prune -f --filter "until=168h" || true