What it's saying is that you have a container you use only to store the data, then share that container's volume with the target container. Finally, because it's on your hard drive, you can access it.
Basically, instead of host -> container, you do container -> container.
And yeah, the container is supposed to be effectively a new computer, but you can build images to create the auth environments you need and base everything off those.
But that doesn't work unless you do everything with public permissions. Seriously try it. Make a container that you share a volume with. Do something in that container on some data and see who owns it according to your computer.
The use case I have above is a real one. Somehow try to share your .ssh folder with a utility container sometime.
User namespaces somewhat help. But fall flat for the case of "still being you" on a container.
It can be comical even for the intended two containers choice. Unless both use the same uid:gid to write files, one container will not be able to read the other containers data.
So here is how I would think about your problem: am I doing this the way the tool wants me to do this? Does the tool even support this use case? If not, how should it be done? Is it impossible, or do I just not like the aesthetics of the solution?
If it's just the aesthetics of the solution, then you have a way forward. But if it is literally impossible to get data on and off a container, then you have a problem.
Basically, instead of host -> container, you do container -> container.
And yeah, the container is supposed to be effectively a new computer, but you can build images to create the auth environments you need and base everything off those.