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Variables resolved from env_file but not taking effect in docker-compose.yaml

My attempt is similar to this example in documentation. I have an image.env file as shown below.

IMAGE_REPO=repo
IMAGE_NAME=name
IMAGE_TAG=latest

My docker-compose.yaml is as follows.

version: "3.9"

services:
  demo:
    env_file:
      - image.env      
    image: "${IMAGE_REPO}/${IMAGE_NAME}:${IMAGE_TAG}"

A docker-compose config results in the following.

WARNING: The IMAGE_REPO variable is not set. Defaulting to a blank string.
WARNING: The IMAGE_NAME variable is not set. Defaulting to a blank string.
WARNING: The IMAGE_TAG variable is not set. Defaulting to a blank string.
services:
  demo:
    environment:
      IMAGE_NAME: repo
      IMAGE_REPO: name
      IMAGE_TAG: latest
    image: '/:'
    restart: on-failure
version: '3.9'

Note the warnings and also the value of services.demo.image. But, also look at services.demo.environment - they are correct values.

Why are the environment variables taking effect?

The env_file parameter is for defining environment variables inside the running container. It will not be used for variable substitution in your docker-compose.yaml file (as you have seen).

You have two options:

  1. (1) You can run: source image.env before running docker-compose up so that those environment variables are set in your system environment

  2. (2) You can rename image.env to .env and then docker-compose will read them as environment variables to be used for substitution in your docker-compose.yaml file.

I believe fo from your example that you want the second option.

@rofrano indeed, I ended up with option 2. Nevertheless, is there a way to parameterize the compose YAML also? Perhaps, similar to what kustomize does with Kubernetes YAML, etc.?

@nsubrahm Actually in researching this a bit more there is a third option. Instead of renaming image.env to .env you can also tell compose the name of the environment file that you want to use like this:

docker-compose --env-file image.env up 

Which, I believe, answers your follow-on question of how to parameterize the compose YAML similar to what kustomize does with Kubernetes YAML. You can have multiple xxx.env files and run docker-compose passing different ones in with the --env-file option to get similar behavior as kustomize.

Thanks, @rofrano. It will be interesting to know, if I can use both the --env-file (for users to edit) and the default .env file (that I do not want to be edited) with non-overlapping variables. Will set-up some experiments.

I believe that --env-file overrides the .env file. You can have one or the other but not both. So not like kustomize in that respect.

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