Docker Compose Vars

Docker has a somewhat complex relationship with arguments. For starters, one has to distinguish between build-time variables (a.k.a. $ARG in Dockerfile) and runtime variables (a.k.a. $ENV). Then, there are environment variables that are set at runtime, and there are variables that are set in docker-compose.yml files. Finally, there are variables that are set in .env files.

For one of my services, through trial and error I found a horribly inefficient way of passing data, which involved combination of both $ARGs and $ENVs. It was a mess, and I was not happy with it. Worst of all (and I found it out only accidentally) that I was passing some tokens into the built image; that was the last straw.

This will serve more like a note to myself than anything else, but I hope it will be useful to someone else.

Dockerfile:

FROM alpine

CMD ["sh", "-c", "echo \"Hello world, $USERNAME\""]

docker-compose.yml:

version: '3.7'

services:
  my-service:
    build: .
    environment:
      - USERNAME=${USERNAME}

basic.env:

USERNAME=sgzmd

To run this abomination, one would have to do:

docker-compose --env-file basic.env up

It would print something like that:

[+] Running 1/0
⠿ Container dockertest-my-service-1  Recreated   0.0s
Attaching to dockertest-my-service-1
dockertest-my-service-1  | Hello world, sgzmd
dockertest-my-service-1 exited with code 0

This is a very basic example, but it shows how to pass a variable from the host environment into the container. Hope this helps someone (and I’ll be definitely fixing my own service soon).