Allow forcing overlayfs

[pschichtel]

Currently podman/buildah won't attempt to mount native overlay filesystems for images, by first probing for the backing filesystem. This is probably very reasonable in most setups, however when using sysbox as the container runtime, this leads to podman being unable to use sysbox' overlayfs support. If I understand their description correctly, they intercept syscalls and make sure the mount goes to a dynamic per-container path that is not on an overlayfs.

Sysbox' documentation on the topic: https://github.com/nestybox/sysbox/blob/46ba726e8e894aa22e20465a32d22dfa2863ec12/docs/user-guide/design.md#overlayfs-mounts-inside-the-sysbox-container

This issue came up when running podman via gitlab-ci runners using the docker executor with the sysbox runtime.

I propose 2 solutions:

1. Change the storage driver to actually attempt to mount using the native overlayfs and only when that fails, fallback to fuse and eventually fail completely
2. Add a configuration option to force the use of native overlay driver.

[pschichtel]

this can easily be reproduced:

1. install and start sysbox
2. configure docker to explicitly use overlayfs (might default to e.g. btrfs depending on the /var/lib filesystem) and configure the sysbox-runc runtime as documented by sysbox
3. run docker run --runtime sysbox-runc --rm -it quay.io/containers/podman:latest
4. run podman run --rm -it alpine:latest inside that container