Would start bash in the container, but also layer into the filesystem all the rest of a standard ubuntu image only for my tools, but not affecting the application in the running container.
I'm pretty sure that's possible with current linux kernel mount namespace/overlayfs infrastructure used by docker - all that's needed is the command line tool to support it.
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.
I'm pretty sure that's possible with current linux kernel mount namespace/overlayfs infrastructure used by docker - all that's needed is the command line tool to support it.