Dockerfile Build Errors With React Static Site In App Platform

Problem I’m using App Platform to serve a React (using Create React App) static website. I’m using Docker in order to have predictable and abstracted builds (as well as other reasons)

But whenever I try to deploy it to DO’s App Platform I get build errors essentially saying that it can’t find the output directory. Such as ‘client | 19:44:34 ! /app/build does not exist within the built container’

App.yaml

name: project_name

static_sites:
  - name: client
    dockerfile_path: ./client/Dockerfile
    github:
      repo: valencian-digital/unique-expressions
      branch: master
    source_dir: client
    output_dir: /app/build

Dockerfile

FROM node:14-alpine

WORKDIR /app
VOLUME /app

ENV NODE_ENV=production

COPY . .

CMD ["npm", "run", "build"]

Question How can I get a Dockerfile to work property with App Platform static sites?