[FEAT] Add Docker tmpfs Mount

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Is your feature request related to a problem? Please describe.

I get frustrated when I know a node operator or Flux administrator can snoop on my files in a container, easily.

This feature is useful to temporarily store sensitive files that you don't want to persist in either the host or the container writable layer. - (Taken directly from the Docker tmpfs mounts page)

Describe the solution you'd like

Their are two options that you can choose from to implement.

You can find them here: https://docs.docker.com/storage/tmpfs/#use-a-tmpfs-mount-in-a-container

Describe alternatives you've considered

Their aren't really any viable alternatives being that FluxOS control how a container is started.

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The solution I provided above would allow a user to create their app in a way that only would use the temporary RAMDISK directory that would be constructed as a result of starting the Docker container with either of those flags specified in the link I gave you.

Another option that I tested today that eliminates the ability to access any of the docker containers from a disk and only store those files in RAM is to run this command: mount -t tmpfs -o size=1024m tmpfs /var/lib/docker

The size parameter would need to specified accordingly. Though not sure how it would be able to be implemented in Flux

EDIT: This seems to be very interesting and would (I think) accomplish the same type of behavior.

(First Comment)
moby/moby#41110

(Edits that were made to accomplish)
moby/moby@master...Jongy:moby:overlayfs-upperdir-tmpfs