How does one add binarys or buildpacks to the image used by the Apps platform?

I am trying to build a PHP worker that has the ffmpeg binary installed to transcode a video. I have included the ffmpeg composer extension so I can access the library but don’t understand how I can adjust what is installed on my worker container.

In another community post it seemed like your could build your own dockerbuild file and that would get you most of what was needed, but I didn’t really follow how that worked on the app platform.

HALP.