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

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.

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.