Docker: fix broken build_static

docker-compose.yml

@@ -20,6 +20,7 @@ services:
     command: bash -c "python manage.py migrate && gunicorn -b 0.0.0.0:8000 --capture-output --error-logfile - --access-logfile - spejstore.wsgi:application"
     volumes:
       - .:/code
+      - /code/build_static
     ports:
       - "8000:8000"
     depends_on: