chore: switch to nextjs user in Dockerfile and update start.sh to run frontend in background

Dockerfile

@@ -54,6 +54,9 @@ RUN mkdir -p /app/generated_images && \
 COPY start.sh .
 RUN chmod +x start.sh
 
+# Switch to nextjs user
+USER nextjs
+
 EXPOSE 3000 8000
 
 CMD ["./start.sh"]

start.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Start frontend as nextjs user
-su nextjs -c "node frontend/server.js" &
-# Start backend as nextjs user
-su nextjs -c "python -m uvicorn backend.app.main:app --host 0.0.0.0 --port 8000"
+# Start frontend in background
+node frontend/server.js &
+# Start backend
+python -m uvicorn backend.app.main:app --host 0.0.0.0 --port 8000