First: This is an unsupported install. Mostly, if you’re not running a standard docker install, you’re on your own.
But that said, here is what I think you need to know:
You need to pass all of those ENV variables to your ECS containers. You can use ./launcher start-cmd to get it to print that stuff. The app.yml is used just to build the image. Those variables aren’t embedded in the container. I am pretty sure that if you do whatever to get those ENV variables passed to your container it’ll do what you want.
I’ve never done an ECS install, but it looks like I will soon. I’d be happy to know whether that works.
You also asked the same thing here: Aws deployed discourse is fetching assets from database url - #18 by trying2survive and got the same answer. 