Pass environment variable to dockerfile build - docker

Question

I am trying to create a service consisting of a web server and database all in docker containers. Currently I am trying to create same environment file for both of them that would contain database credentials. Unfortunatelly, when I try to build database with it, it turns out that they are empty. How can I create a project with single environment file for both components? Here is my docker-compose.yml:
version: '2'
services:
db:
build:
context: .
dockerfile: Dockerfile-db
ports:
- '5433:5432'
env_file:
- env
web:
build: .
ports:
- '8000:8000'
command: venv/bin/python manage.py runserver 0.0.0.0:8000
depends_on:
- db
env_file:
- env
Here is part of my Dockerfile-db file responsible for creating database:
FROM ubuntu:16.04
RUN apt-get update && apt-get install -y postgresql-9.5-postgis-2.2
USER postgres
ARG DB_PASSWORD
ARG DB_NAME
RUN echo $DB_PASSWORD
RUN /etc/init.d/postgresql start && \
psql --command "ALTER USER postgres WITH PASSWORD '$DB_PASSWORD';" && \
psql --command "CREATE DATABASE $DB_NAME;" && \
psql --command "\\c $DB_NAME" && \
psql --command "CREATE EXTENSION postgis;" && \
psql --command "CREATE EXTENSION postgis_topology;"
And my env file has following structure:
DB_NAME=some_name
DB_USER=postgres
DB_PASSWORD=some_password
DB_HOST=db
DB_PORT=5432

Answer 1

The environment file is not part of the build process, it is used when running the container.
You need to use build-args. In docker-compose you can specify build args in the file:
build:
context: .
dockerfile: Dockerfile-db
args:
DB_NAME: some_name
DB_USER: postgress
...
This might not be a good idea if you want to publish the composefile, as you are storing credentials in it. You can explicitly build and pass --build-arg
docker-compose build --build-arg DB_NAME= some_name ...
And when running specify no build in docker-compose run --no-build
Update:
As suggested by #gonczor, a shorter and cleaner syntax to use pass the env file as build args is:
docker-compose build --build-args $(cat envfile)

Answer 2

Your configuration is correct, it seems like you may not be passing the actual path or the name of the .env file. Have you tried the following (assuming your .env file is in the same directory).
version: '2'
services:
db:
build:
context: .
dockerfile: Dockerfile-db
ports:
- '5433:5432'
env_file:
- ./.env
web:
build: .
ports:
- '8000:8000'
command: venv/bin/python manage.py runserver 0.0.0.0:8000
depends_on:
- db
env_file:
- ./.env