How to use npm and node.js (to build js static files) in Python Image for tests purpose?

Hello guys! I have python project with reactjs on front end. As well I have webpack script that builds static files for python project. As well few python tests relay on static files, they tests whether server services those files right. So what is the right way to write CI script for testing back end?

test:backend:
  image: python:2.7
  stage: test
  before_script:
    - pip install -r requirements.txt
  script:
    - py.test --cov app --cov-report term-missing)

So I’d like to have something like that:

test:backend:
  image: python:3.5
  stage: test
  before_script:
    - pip install -r requirements.txt
  script:
    - (cd client && npm install && npm run build:dev)
    - py.test --cov app --cov-report term-missing)

but it doesn’t work because we don’t have node and npm in python image. As I know according to Docker documentation we should have only one process in one Image https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/run-only-one-process-per-container. Could I use Services of something to launch npm run build? What is the best practice for that and how can I archive result in gitlab CI?