I’ve been on support thread with the wonderful team at Gigalixir. So the documentation states that you simply place ./assets/package.json in the root of your repo.

{
  "scripts": {
    "deploy": "cd ../ && mix assets.deploy && rm -f _build/esbuild*"
  }
}

However, since this was an umbrella app I assumed it shoul go into the root of the web application. It would appear that Gigalixir always look for the existence of ./assets/package.json in the root of the repo. But I needed the script to be run in the context of the nested web application.

{
  "scripts": {
    "deploy": "cd ../apps/my_umbrella_app_red_web && mix assets.deploy && rm -f _build/esbuild*"
  }
}

Problem solved, and now I realise I can ssh to the container that was good to confirm the assets actually built at all and ensure they are built in the correct location.

Thanks Gigalixir.