Add build stanza to migration

docker/docker-compose.prod.yml

@@ -20,6 +20,7 @@ services:
 
   migration:
     image: pathogen/neptune:latest
+    build: ./docker/prod
     command: sh -c '. /venv/bin/activate && python manage.py migrate --noinput'
     volumes:
       - ${PORTAINER_GIT_DIR}:/code