How to use compose environment variables in docker ENTRYPOINT

Question

Environment variables don't appear to work in ENTRYPOINT . It is my understanding that the shell form of ENTRYPOINT will expand ENV variables at run time, but this doesn't to appear to work for ENV_CONFIG_INT in the example below. What have I done wrong in the following example?

Dockerfile

ENTRYPOINT [ "yarn", "run", "app-${ENV_CONFIG_INT}" ]

Compose yaml

test:
    image: testimage/test:v1.0
    build:
        context: .
        dockerfile: Dockerfile
    env_file:
        - ./docker.env
    environment:
        - ENV_CONFIG_INT=1

Error:

error Command "app-${ENV_CONFIG_INT}" not found.

Replacing the value with a static int of say 1 fixes the issue, however I want the value to be dynamic at runtime.

Thanks in advance.

Answer 1

I wouldn't try to use an environment variable to specify the command to run. Remove the line you show from the Dockerfile, and instead specify the command: in your docker-compose.yml file:

test:
  image: testimage/test:v1.0
  build: .
  env_file:
    - ./docker.env
  command: yarn run app-1  # <--

As you note, the shell form of ENTRYPOINT (and CMD and RUN ) will expand environment variables, but you're not using the shell form: you're using the exec form , which doesn't expand variables or handle any other shell constructs. If you remove the JSON-array layout and just specify a flat command, the environment variable will be expanded the way you expect.

# Without JSON layout
CMD yarn run "app-${ENV_CONFIG_INT:-0}"

(I tend to prefer specifying CMD to ENTRYPOINT for the main application command. There are two reasons for this: it's easier to override CMD in a plain docker run invocation, and there's a useful pattern of using ENTRYPOINT as a wrapper script that does some initial setup and then runs the CMD .)

Answer 2

The usual way to ensure that environment variable expansion works in the entrypoint or command of an image is to utilize bash - or sh - as the entrypoint.

version: "3.8"

services:
  test:
    image: alpine
    environment:
      FOO: "bar"
    entrypoint: ["/bin/sh", "-c", "echo $${FOO}"]

$ docker-compose run test
Creating so_test_run ... done
bar

The other thing you need to do is properly escape the environment variable, so its not expanded on the host system.