Dockerfile - use environment variable from env file - docker

Question

I have a docker compose setup where I want to use environment variables from env file in my dockerfile. I want to use these variables during the build time since I use this version number in concatenating the string in order to form a download URL.
Here I wrote part of the files I'm using just to keep the focus on the point of my question.
.env
MY_APP_VER=v1.2.3
docker-compose.yml
version: "2"
services:
my-app:
build: .
container_name: my_app
environment:
- my_app_version=$MY_APP_VER
Dockerfile
FROM scratch
ENV my_app_ver=$my_app_version
RUN echo $my_app_ver
I have checked various sources but without any success. I'm not sure if this is even possible or am I using the wrong syntax (should I use quotes or no e.g. "$my_app_ver" or curly brackets ${my_app_ver}).

Answer 1

For version 3.8 you can do it in the following way
version: '3.8'
services:
my-app:
build: .
ports:
- ${CONTAINER_PORT}:${PORT} # for example
env_file: .env
container_name: my-app-${NODE_ENV} # for example
environment:
MYSQL_DATABASE: ${DB_NAME} # for example
my_app_version: ${MY_APP_VER} # for your case
Find more information in documentation
Also, you can find more information about the usage of env variables in Dockerfile and docker-compose here

Answer 2

There is an option called env-file in docker-compose, that you can leverage: https://docs.docker.com/compose/environment-variables/#the-env_file-configuration-option
version: "3"
services:
my-app:
build: .
container_name: my_app
env_file:
- .env.dev
Be aware, that the .env file is loaded by default, if it is present in the current context. So you only have to use env_file, if it is named differently or is in a different folder.