Where to run collectstatic when deploying django app to heroku using docker?

After confirming with Heroku support, this does indeed appear to be a bit of a catch-22.

The solution was to put collectstatic in the Dockerfile so that it runs during build time and the files persist.

We got around not having a secret key config var by setting a default secret key using the get_random_secret_key function from Django.

The run phase uses the secret key from the Heroku config vars, so we aren't actually changing the secret key every time -- the default only applies to the build process. collectstatic doesn't index on the secret key, so this is fine.

In settings.py

from django.core.management.utils import get_random_secret_key
...
SECRET_KEY = os.getenv('DJANGO_SECRET_KEY', default=get_random_secret_key())

I don't use heroku so can't test, but you should be able to run collect static before you run the app;

Dockerfile

# Pull base image
FROM python:3.7-slim

# Set environment varibles
ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1

# Set work directory
WORKDIR /code/

# Install dependencies
RUN pip install pipenv
COPY Pipfile Pipfile.lock .
RUN pipenv install --system

# Copy project
COPY . .

# Collect static files
RUN python manage.py collectstatic --noinput

# run gunicorn
CMD gunicorn hello_django.wsgi:application --bind 0.0.0.0:$PORT

You could also not run collectstatic in your dockerfile, or event run the application because these can be ran by heroku.yml, for example;

build:
  docker:
    web: Dockerfile
  config:
    DJANGO_SETTINGS_MODULE: project.settings
run:
  web: gunicorn backend.config.wsgi:application --bind 0.0.0.0:$PORT
release:
  image: web
  command:
    - python manage.py collectstatic --noinput

You also shouldn't need to mkdir for your working directory. Just set WORKDIR /code/ early in your dockerfile and after that things will run based on that directory.

There's a decent article on this here; https://testdriven.io/blog/deploying-django-to-heroku-with-docker/