docker-compose.yml

version: "3.4"
services:
  postgres:
    user: "${UID}:${GID}"
    image: postgres:9.6.18-alpine
    expose:
      - "5432"
    ports:
      - "5432:5432"
    volumes:
      - .development_db:/db
    environment:
      POSTGRES_PASSWORD: password
      POSTGRES_USER: user
      POSTGRES_DB: dbname
      PGDATA: /db

  redis:
    image: "redis:alpine"
    ports:
      - "6379"

  web:
    restart: on-failure
    user: "${UID}:${GID}"
    build:
      context: .
      target: development
    environment:
      DATABASE_URL: "postgresql://user:password@postgres:5432/dbname"
      REDIS_CACHE_LOCATION: "redis://redis:6379/1"
    volumes:
      - .:/app
    ports:
      - "8000:8000"
    depends_on:
      - postgres
      - redis

Dockerfile

# Runtime images
FROM python:3.8.8-slim as runtime
EXPOSE 8000
COPY . /app/
RUN mkdir -p /app/static/js
WORKDIR /app
RUN pip install -r requirements.txt
RUN python manage.py collectstatic --no-input --no-color
CMD gunicorn crohn.wsgi:application -w 2 -b :8000

FROM runtime as development
CMD 'bash -c "pip install -r requirements.txt && python manage.py migrate && python manage.py collectstatic --no-input && python manage.py runserver 0.0.0.0:8000"'

Ah, forgot to add the command to run it:

UID=$UID GID=$GID docker-compose up

The reason I'm specifying the UID and GID is so that the containerized processes don't end up littering the directory with files created by root which then become a pain to clean up.