Missing Environment Vars in docker python:3 with docker-compose

Question

Though my configuration looks good, my python:3 image does not seem to have the expected DJANGO_SECRET_KEY set, at least at the point that the Dockerfile attempts to run migrations

$ docker-compose config
services:
  api:
    build:
      context: /Users/ben/Projects/falcon/falcon-backend
      dockerfile: Dockerfile
    depends_on:
    - db
    - redis
    environment:
      DJANGO_SECRET_KEY: 'some-secret-that-works-elsewhere'
$
$ docker-compose up --build api
[...]
 Step 6/7 : RUN echo `$DJANGO_SECRET_KEY`
 ---> Running in fbfb569c0191

[...]
django.core.exceptions.ImproperlyConfigured: Set the DJANGO_SECRET_KEY env variable
ERROR: Service 'api' failed to build: The command '/bin/sh -c python manage.py migrate' returned a non-zero code: 1

however, the final line, CMD python manage.py runserver 0.0.0.0:8001 --settings=falcon.settings.dev-microservice does start up as desired, with the necessary env vars set.

# Dockerfile -- api

FROM python:3

RUN pip3 -q install -r requirements.txt
RUN echo `$DJANGO_SECRET_KEY`
RUN python manage.py migrate --settings=falcon.settings.dev-microservice # <-- why does this not work
CMD python manage.py runserver 0.0.0.0:8001 --settings=falcon.settings.dev-microservice

Why does the penultimate line of the Dockerfile fail due to an unset environment variable while the final one works as expected?

Answer 1

The environment variables not declared inside the Dockerfile are not visible to the container when building the image. They are only passed to the container at runtime. Since the RUN instruction executes on build, the environment variable DJANGO_SECRET_KEY which is declared outside the Dockerfile won't be visible to the RUN command.

To solve the issue you can declare the env variable inside the Dockerfile and set it via a build argument:

FROM python:3

RUN pip3 -q install -r requirements.txt
ARG key
ENV DJANGO_SECRET_KEY=$key
RUN echo `$DJANGO_SECRET_KEY`
RUN python manage.py migrate --settings=falcon.settings.dev-microservice
CMD python manage.py runserver 0.0.0.0:8001 --settings=falcon.settings.dev-microservice

Then, you should modify the composefile as such:

build:
  context: /Users/ben/Projects/falcon/falcon-backend
  dockerfile: Dockerfile
  args:
    - key='secrete-key'