Pass environment variables from docker-compose to container at build stage

Your docker-compose.yml should look like this:

version: '2'
services:
  web:
    build:
      context: ./web
      args:
        REQUIREMENTS: "requirements_dev.txt"

Your Dockerfile should define the build-argument using ARG like this:

FROM python:3.5
ENV PYTHONUNBUFFERED 1
ENV APP_ROOT /usr/src/app
ARG REQUIREMENTS
...
COPY $REQUIREMENTS $APP_ROOT/
RUN pip install -r $APP_ROOT/$REQUIREMENTS

I validated this and created a minified functional demo at Github:

https://github.com/jannikweichert/stackoverflow-41747843