Environment variable not set in container image after PUSH and PULL - docker

Question

I would like to see if we can find a way to persist env variables in images AFTER they are pulled from private docker registry. I have done the following steps
content of .env file
APP_DB_CACHE_UPDATE = 3600
content of docker-compose file
services:
app:
build: .
ports:
- "9000:9000"
environment:
- APP_DB_CACHE_UPDATE=${APP_DB_CACHE_UPDATE}
command to build and check container and its env vars
$ APP_DB_CACHE_UPDATE=3602 docker-compose up
$ docker exec -it <container_id> printenv
APP_DB_CACHE_UPDATE=3602
then I
push the image to private docker registry
I remove image from local machine
pull image from docker registry
$ docker tag app-name_app-name:latest localhost:5000/app-name_app-name:latest
$ docker push localhost:5000/app-name_app-name:latest
$ docker rmi localhost:5000/app-name_app-name:latest
$ docker pull localhost:5000/app-name_app-name:latest
and now I check the enviorment after running this image, i am unable to see APP_DB_CACHE_UPDATE env variable
docker exec -it <new container image> printenv

Answer 1

Docker compose does not build images with env vars set through "environment" or "env_file". It builds images first, then provides the environment variables to the container runtime. See environment specification:
If your service specifies a build option, variables defined in
environment are not automatically visible during the build.
Because of this, your image does not know about the environment variable specified in docker-compose file.
To set environment variables during/before the build, which will be persisted in the container after the image is built, you need to specify them in you build environment. You can do this in Dockerfile using ENV, for example:
ENV APP_DB_CACHE_UPDATE="3600"
See Dockerfile ENV specification.
The environment variables set using ENV will persist when a container
is run from the resulting image. You can view the values using docker
inspect, and change them using docker run --env =
Then tell compose to use your Dockerfile with:
build:
context: .
dockerfile: Dockerfile