Add NPM_REGISTRY build arg support in Dockerfile

- Accept NPM_REGISTRY ARG in frontend build stage
- Configure npm registry before install if provided
- Add build-arg to MR build stage in CI

Dockerfile

@@ -1,8 +1,13 @@
 # Frontend build stage
 FROM node:20-alpine AS frontend-builder
 
+ARG NPM_REGISTRY
+
 WORKDIR /app/frontend
 
+# Configure npm registry if provided
+RUN if [ -n "$NPM_REGISTRY" ]; then npm config set registry "$NPM_REGISTRY"; fi
+
 # Copy package files
 COPY frontend/package*.json ./
 RUN npm install