Environment Variables in Docker Recap

Modern application development favors separating configuration from code. By using environment variables—especially within Docker containers—you can adhere to The Twelve-Factor App methodology and deploy more flexibly.

Why Use Environment Variables?
Step 1: Read Environment Variables Locally
Step 2: Pass Variables into Docker
Docker Commands Reference
Further Reading

Why Use Environment Variables?

Hardcoding configuration values (such as colors, database URLs, or API keys) leads to brittle deployments and requires code changes for every tweak. Environment variables allow you to:

Decouple code from deployment details
Support multiple environments (dev, staging, prod) without modifying source
Secure sensitive data outside of version control

Using environment variables is a best practice for twelve-factor compliant apps.

Step 1: Read Environment Variables Locally

Below is a simple Flask application (app.py) that reads APP_COLOR from the environment, defaulting to "red" if unset.

import os
from flask import Flask, render_template

app = Flask(__name__)

# Read background color from environment; default to "red"
color = os.environ.get("APP_COLOR", "red")

@app.route("/")
def main():
    print(f"Current color: {color}")
    return render_template("hello.html", color=color)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080)

To test locally:

export APP_COLOR=blue
python app.py

Visit http://localhost:8080—the page background will reflect your APP_COLOR. If you don’t set APP_COLOR, it gracefully falls back to red.

Never commit sensitive environment variables (like credentials) to your code repository. Consider using a secrets manager for production.

Step 2: Pass Variables into Docker

Once your app is containerized, leverage Docker’s -e flag (or --env-file) to inject runtime configuration.

Build the image
docker build -t simple-webapp-color .

Run with a custom color

docker run -e APP_COLOR=blue simple-webapp-color

Scale with different settings

docker run -e APP_COLOR=green simple-webapp-color
docker run -e APP_COLOR=yellow simple-webapp-color

By externalizing APP_COLOR, you keep one image for all environments—no code changes required.

Docker Commands Reference

Command	Purpose	Example
`docker build`	Build an image from a `Dockerfile`	`docker build -t simple-webapp-color .`
`docker run -e VAR=VALUE`	Run a container with an environment var	`docker run -e APP_COLOR=blue simple-webapp-color`
`docker run --env-file ./env.list`	Load multiple vars from a file	`docker run --env-file ./env.list simple-webapp-color`

Watch Video

Commands and Arguments in Kubernetes

Environment Variables

Introduction

Docker Engine

Docker Image Management

Docker Engine - Security

Docker Engine - Networking

Docker Engine - Storage

Docker Compose

Docker Swarm

Kubernetes

Docker Engine Enterprise

Docker Trusted Registry

Disaster Recovery

Environment Variables in Docker Recap

Table of Contents

Why Use Environment Variables?

Step 1: Read Environment Variables Locally

Step 2: Pass Variables into Docker

Docker Commands Reference

Further Reading

Watch Video

​Table of Contents

​Why Use Environment Variables?

​Step 1: Read Environment Variables Locally

​Step 2: Pass Variables into Docker

​Docker Commands Reference

​Further Reading

Watch Video

Table of Contents

Why Use Environment Variables?

Step 1: Read Environment Variables Locally

Step 2: Pass Variables into Docker

Docker Commands Reference

Further Reading