vishnu-13-97/mern-docker-compose ? reverse-engineered prompt

Build me a simple MERN app that runs fully in Docker.

I want a React frontend, a Node and Express backend, and MongoDB as the database, all wired together with Docker Compose so I can start everything with one command. The frontend should talk to the backend through the right local URL, and the backend should connect to MongoDB through the container network. Make it easy to run and easy to understand, with clear setup instructions and sensible defaults.

Please include the Docker files and compose setup needed for development, plus any app code needed so the whole thing actually works end to end. If you need to check current best practices or docs while building it, go ahead and look them up.