And crucially, once you have given a resource limit, there's no way to (trivially) feed that back into an environment variable or flag to signal that to the app

And crucially, once you have given a resource limit, there's no way to (trivially) feed that back into an environment variable or flag to signal that to the app runtime (which, IIRC, is Really Handy for Java-based apps and can seriously improve the performance of Go-based ones).