diff --git a/Dockerfile b/Dockerfile
index 724f2a7..8765afd 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -5,6 +5,9 @@ FROM node:20-alpine AS builder
 
 WORKDIR /app
 
+# Install Yarn 1 (classic) – ensures compatibility with existing lockfile
+RUN npm install -g yarn@1.22.19 && yarn --version
+
 # Install dependencies (cache layer)
 COPY package.json .
 COPY yarn.lock .