Help, deploy error finding executable heroku-php-nginx

I am trying to create a new app using the App Platform which is the built and deployed the same as an existing app in our account, created by a legacy developer.

It is a PHP app, built from a Docker image. On deploy I am getting an error from the Run Command as follows:

starting container: starting non-root container [heroku-php-nginx -C server/digitalocean/nginx_app.conf public/]: error finding executable "heroku-php-nginx" in PATH [/opt/bitnami/php/bin /opt/bitnami/php/sbin /usr/local/sbin /usr/local/bin /usr/sbin /usr/bin /sbin /bin]: no such file or directory

My understanding is the Heroku buildpacks are included in the app creation, but perhaps that is not the case if building from a Dockerfile?

However, there is no indication of any Heroku build PHP/Nginx reference in the existing Dockerfile, and the above is the Run command from the legacy app.

So I am a bit stumped here as to what to do. Should the buildpacks be added to Docker since I am going that route to build the app? If so some guidance would be much appreciated.