Update: I solved this eventually by setting the SHOPIFY_API_KEY in the pipeline. It turned out that the front-end was not being built at all and so couldn’t be found. Look out for this message in the build logs:
“Building the frontend app without an API key. The frontend build will not run without an API key. Set the SHOPIFY_API_KEY environment variable when running the build command.”
You can set this in the pipeline or install dotenv and pop it a .env file