How to use compose environment variables in docker ENTRYPOINT - docker

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.