The "It Works On My Machine" Illusion You've built a beautiful, decoupled microservices architecture. Your Node/Express API runs smoothly on port 3000, and your React frontend fetches data perfectly from http://localhost:3000/api . You containerize both services, spin them up with Docker Compose, and suddenly the frontend crashes with a Connection Refused error. What happened? You fell into the localhost trap. Understanding Docker's Internal Network When running services locally without Docker, localhost refers to your host machine. Both your frontend and backend share this environment. However, when you containerize these services, Docker creates an isolated virtual network. Inside a container, localhost no longer refers to your laptop; it refers to the container itself. When your React container tries to fetch from localhost:3000, it is looking inside its own isolated environment, where the Node API does not exist.…