The new ephemeral container support in kubernetes lets you do essentially that. You bring the filesystem from another container image into the PID/network namespace of a running container in a pod.

lol, it's fun to imagine going back in time to explain what you just said to my 2004 sysadmin self. back when I used to build servers, and colo them, and physically maintain them.

Which version of bash would you expect to run in this example?

(1) If it's the bash version from the standard ubuntu image, you will need to specify where to mount your application's filesystem inside the ubuntu filesystem.

(2) If it's the bash version from your application, then it's the other way around: you will need to specify where to mount the ubuntu filesystem inside your container.

Option (1) seems more practical. My point is that you will need to specify a mountpoint either way, and your commands will need to take this mountpoint into account.

Mount both filesystems at the same place as an overlay - if a file exists in both, I don't care which I see.

I see. That will work as long as your app is built on the same distro as your tools image (in your case, ubuntu).