Pass variables from .env file to dockerfile through docker-compose

Question

project
└───app
│   │   ...
│   │   Dockerfile
│   │
└───prod.env
└───docker-compose.yml

My docker-compose looks like this:

services:
   app:
      build:
         context: .\app
         args:
            ARG1: val1
            ARG2: val2
      env_file:
         - prod.env

But I've tried this too:

services:
   app:
      build:
         context: .\app
         args:
            ARG1: ${ARG1}
            ARG2: ${ARG2}
      env_file:
         - prod.env

My prod.env file looks like this:

ARG1 = 'val1'
ARG2 = 'val2'

But I've tried this too:

ARG1=val1
ARG2=val2

I would like for either the values of args or the values from the prod.env file to be passed to the dockerfile.

This is what I've tried to get this:

ARG ARG1
ARG ARG2

RUN echo ${ARG1}
RUN echo ${ARG2}

ENV ARG1 ${ARG1}
ENV ARG2 ${ARG2}

RUN echo ${ARG1}
RUN echo ${ARG2}

ENV ARG1 "new val2"
ENV ARG2 "new val2"

RUN echo ${ARG1}
RUN echo ${ARG2}

It always end with blank values.

Any help would be greatly appreciated. I feel like no answers from other posts have worked when I tried them.

To build I use docker-compose --env-file prod.env build

Thanks

Update Sergio Santiago asked if I could run docker-compose config and show the results.

Here are the final files I used for this test.

docker-compose:

services:
   app:
      build:
         context: .\app
         args:
            ARG1: val1
            ARG2: val2
      env_file:
         - prod.env

prod.env:

ARG3 = 'val3'
ARG4 = 'val4'

And here is the output of docker-compose --env-file prod.env config

networks:
  demo-net: {}
services:
  app:
    build:
      args:
        ARG1: val1
        ARG2: val2
      context: C:\project\app
    environment:
      ENV: prod.env
      ARG3: val3
      ARG4: val4

I would like to add that clearly from here getting the variable from the .env file to the docker-compose file is not the issue. I also have a flask app running on the container and through os.environ it is able to use the variables in the .env file. I just can't figure out how to give the same access to the Dockerfile.

Update 2 More specific information in relation to ErikMD's answer

prod.env

DOMAIN = 'actualdomain.com'
ENV = 'prod.env'
ENV_NUM = 1
ARG1 = 'value1'

dev.env

DOMAIN = 'localhost'
ENV = 'dev.env'
ENV_NUM = 0
ARG1 = 'value1'

Notice that the value for ARG1 is the same but the other values are different.

docker-compose.yml

version: "3.7"
services:
  home:
    image: home-${ENV_NUM}
    build: 
      context: .\home
      args:
        ARG1: "${ARG1}"
    networks:
      - demo-net
    env_file:
      - ${ENV}
    labels:
      - traefik.enable=true
      - traefik.http.routers.home.rule=Host(`${DOMAIN}`)
      - traefik.http.routers.home.entrypoints=web
    volumes:
      - g:\:c:\sharedrive
...
...
  reverse-proxy:
    restart: always
    image: traefik:v2.6.1-windowsservercore-1809
    command:
      - --api.insecure=true
      - --providers.docker=true
      - --entrypoints.web.address=:80
      - --providers.docker.endpoint=npipe:////./pipe/docker_engine
    ports:
      - 80:80
      - 443:443
      - 8080:8080
    networks:
     - demo-net
    volumes:
      - source: \\.\pipe\docker_engine\
        target: \\.\pipe\docker_engine\
        type: npipe
networks:
  demo-net:

The dots represent other apps that would be formatted the same as home.

dockerfile

FROM python:3.10.3

ARG ARG1="default"

ENV ARG1="${ARG1}"

WORKDIR /app

ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1

RUN echo "This is argument 1 -> ${ARG1}"

output of docker-compose --env-file prod.env config

networks:
  demo-net: {}
services:
  home:
    build:
      args:
        ARG1: value1
      context: C:\MIS-Web-App\home
    environment:
      DOMAIN: actualdomain.com
      ENV: prod.env
      ENV_NUM: '1'
      ARG1: value1
    image: home-1
    labels:
      traefik.enable: "true"
      traefik.http.routers.home.entrypoints: web
      traefik.http.routers.home.rule: Host(`mis.canaras.net`)
    networks:
      demo-net: null
    volumes:
    - g:\:c:\sharedrive:rw
...
...

Then I run either docker-compose --env-file prod.env build or docker-compose --env-file dev.env build

output of build

Step 9/23 : RUN echo "This is argument 1 -> ${ARG1}"
 ---> Running in 5142850de365
This
is
argument
1
->
Removing intermediate container 5142850de365

Now I call pass the env_file in the command as well as in the actual file because there are variables in there that my docker-compose file needs and variables that my flask app needs. And there is definitely overlap.

Getting the values from the prod.env or dev.env files to docker-compose is not the issue. Neither is getting it to my flask app. The issue is getting those values to the dockerfile.

Answer 1

My solution was annoying which is why it took me so long to figure it out. My dockerfile was using powershell on a windows server, so I had to do this for every argument:

ARG ARG1
RUN echo $env:ARG1

This seems pretty niche especially since using windows containers on a windows server is not my first choice, so check out @ErikMD 's answer if your having issues with env files and whatnot.