In order to run in dev mode you need to run both services and then place an nginx in front of them.
Open a terminal and run the backend:
cd digital-signature-backend
PORT=3001 go run cmd/digital-signature-backend/main.go
Backend will be listening on default port 3001
Open another terminal and run the frontend
cd digital-signature
npm run dev
Frontend will be listening on default port 3000
Run Nginx load balancer
docker run -p 3002:80 -v $(pwd)/local-dev/nginx.conf:/etc/nginx/nginx.conf:ro nginx
Application will be listening on port 3002