diff --git a/Dockerfile b/Dockerfile
index a08856c6d00b..4bb4d1073c36 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,8 +72,7 @@ RUN --mount=type=cache,id=openclaw-pnpm-store,target=/root/.local/share/pnpm/sto
     NODE_OPTIONS=--max-old-space-size=2048 pnpm install --frozen-lockfile \
       --config.supportedArchitectures.os=linux \
       --config.supportedArchitectures.cpu="$(node -p 'process.arch')" \
-      --config.supportedArchitectures.libc=glibc && \
-    pnpm store add source-map@0.6.1
+      --config.supportedArchitectures.libc=glibc
 
 # pnpm v10+ may append peer-resolution hashes to virtual-store folder names; do not hardcode `.pnpm/...`
 # paths. Matrix's native downloader can hit transient release CDN errors while
@@ -122,7 +121,10 @@ RUN pnpm_config_verify_deps_before_run=false pnpm qa:lab:build
 FROM build AS runtime-assets
 ARG OPENCLAW_EXTENSIONS
 ARG OPENCLAW_BUNDLED_PLUGIN_DIR
+# BuildKit cache mounts are not part of cached layers; seed prune-only
+# tarballs in the same step that runs offline prune.
 RUN --mount=type=cache,id=openclaw-pnpm-store,target=/root/.local/share/pnpm/store,sharing=locked \
+    pnpm store add source-map@0.6.1 && \
     CI=true pnpm prune --prod \
       --config.offline=true \
       --config.supportedArchitectures.os=linux \