fix: ensure public dir exists in developer-portal build

Next.js standalone COPY fails when no public directory exists in source.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

developer-portal/Dockerfile

@@ -12,6 +12,9 @@ RUN npm install
 # Copy source code
 COPY . .
 
+# Ensure public directory exists (may not have static assets)
+RUN mkdir -p public
+
 # Build the application
 RUN npm run build