Docker Environment variables

Environment variables configure containers at runtime. Set them in the Dockerfile with ENV or at runtime with docker run -e. Use .env files with docker compose env_file. Containers inherit the environment from their base image plus any added with ENV or -e.

# Dockerfile
ENV NODE_ENV=production
ENV PORT=3000
 
# At runtime
$ docker run -e NODE_ENV=development myapp                # override ENV
$ docker run -e DEBUG=true -e LOG_LEVEL=info myapp        # multiple variables
$ docker run --env-file .env myapp                        # load from .env file
 
# docker-compose.yml
services:
  app:
    environment:
      - NODE_ENV=production
      - PORT=3000

Variables set in the Dockerfile are baked into the image; variables set with -e or in Compose only exist at runtime. Environment variables are a standard way to configure apps without modifying code—use them for feature flags, database URLs, log levels, and other runtime settings. Compose loads variables from env_file before evaluating the environment section.