August Feng

Environment variables and volumes when building images with Docker Compose

About

Just a short article documenting an investigation on images built from docker compose files.

Code

  • Dockerfile

      FROM alpine

  CMD ["printenv", "ls -l /foobar"]
  • docker-compose.yaml
    services:
      foobar:
        image: helloworld
        build:
          context: .
        volumes:
        - "./foobar:/foobar"
        environment:
        - foobar=helloworld

Run

This is the output when we run docker compose up --build:

  docker-compose0-foobar-1  | foobar=helloworld
  docker-compose0-foobar-1  | HOSTNAME=207ea31f2156
  docker-compose0-foobar-1  | SHLVL=1
  docker-compose0-foobar-1  | HOME=/root
  docker-compose0-foobar-1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  docker-compose0-foobar-1  | PWD=/
  docker-compose0-foobar-1  | total 0
  docker-compose0-foobar-1  | -rw-r--r--    1 root     root             0 Apr  3 16:53 helloworld.txt
  docker-compose0-foobar-1 exited with code 0

This is the output when we run docker run -it helloworld:

  HOSTNAME=00e878cd8bfb
  SHLVL=1
  HOME=/root
  TERM=xterm
  PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  PWD=/
  ls: /foobar: No such file or directory

Conclusion

The environment variable and volume configuration are not relevant for the image building process.