Containerizing a Flask app with Redis and Postgres: multistage builds and secrets

[NicholasVG]

I'm a junior developer tasked with containerizing our legacy Python Flask application using Docker for the first time, and while I've got a basic Dockerfile working, I'm struggling with best practices for multi-stage builds, managing environment variables securely, and orchestrating multiple containers for the app, a Redis cache, and a Postgres database. I've followed a few introductory tutorials, but they often skip the production-grade concerns. For engineers who have deployed Dockerized applications to a cloud environment, what are the key steps and configuration details you wish you knew when you started, particularly around security, logging, and efficient image sizing?