Docker, Environment Variables, and You

This repository demonstrates seven (7) different ways that env vars can be injected into docker, either at buildtime or runtime, each in surprising and not at all irritating ways.

• ENV instruction in Dockerfile (cannot be directly overriden at build time)
• ARG instruction in Dockerfile (can override ENV at build time but only if assigned to do so)
• environment in compose.yml (only evaluated at runtime)
• args in compose.yml (can override Dockerfile instructions)
• .env file
  • can override args during build if explicitly assigned from environment in compose.yml, e.g.

      args:
        NODE_ENV: ${NODE_ENV:-buildtime} # note posix fallback also works here
    

• --build-arg CLI option used with docker compose CLI for build command, e.g.

    docker compose build --build-arg NODE_ENV=peekaboo
  

  this will override .env file
• -e CLI option used with docker compose CLI for run command, e.g.

    docker compose run -e NODE_ENV=gotcha vartest
  

  this will override any settings for an env var at runtime only

License

CC0