add superuser creation command and update settings for environment variables

docker-compose.yml

@@ -0,0 +1,19 @@
+services:
+  web:
+    build: .
+    command: >
+      sh -c "python manage.py migrate &&
+             python manage.py createsuperuser_if_not_exists &&
+             python manage.py collectstatic --noinput &&
+             gunicorn mail_management.wsgi:application --bind 0.0.0.0:8000"
+    volumes:
+      - static_volume:/app/staticfiles
+      - media_volume:/app/media
+    ports:
+      - "8000:8000"
+    env_file:
+      - .env
+
+volumes:
+  static_volume:
+  media_volume: