persist ISR cache between containers

Dockerfile

@@ -26,4 +26,40 @@ COPY --from=builder /app/public ./public
 COPY --from=builder /app/.next/standalone ./
 COPY --from=builder /app/.next/static ./.next/static
 
-CMD ["node", "server.js"]
+RUN <<EOT
+# Mount "/app/.next" to a persistent volume, to persist ISR cache between containers
+set -eux
+mkdir /data
+mv .next /data
+
+cat << 'EOF' > entrypoint.sh
+set -e
+dst=/app/.next
+src=/data/.next
+dst_id_file="$dst/BUILD_ID"
+src_id_file="$src/BUILD_ID"
+
+img_build_id="$(cat "$src_id_file")"
+if [ -f "$dst_id_file" ]; then
+  old_build_id="$(cat "$dst_id_file")"
+else
+  old_build_id="<none>"
+fi
+
+if [ "$img_build_id" != "$old_build_id" ]; then
+  echo "[INIT] Build ID changed, recreating .next files"
+  echo "[INIT] Existing build ID at $dst_id_file: $old_build_id"
+  echo "[INIT] Image build ID: $img_build_id"
+
+  rm -rf "$dst/*"
+  cp -rp "$src/*" "$dst"
+else
+  echo "[INIT] Build ID unchanged, reusing existing .next files"
+  echo "[INIT] Build ID: $img_build_id"
+fi
+
+exec node server.js
+EOF
+EOT
+
+CMD ["entrypoint.sh"]