If you are using a Dockerfile to deploy your app on App Platform, environment variables are only available at build time if you have set them using the --build-arg option in Docker before deploying the app.
For example, to access the variable EXAMPLE=your-value during build time, set it in the image using the docker CLI like this:
docker build --build-arg EXAMPLE=your-value
You cannot use this method to access bindable variables at build time. If you are supplying a Dockerfile, bindable variables are only available at runtime.