Incus vulnerable to local privilege escalation through custom storage volumes

Incus is a system container and virtual machine manager. An issue in versions prior to 6.0.6 and 6.19.0 affects any Incus user in an environment where an unprivileged user may have root access to a container with an attached custom storage volume that has the `security.shifted` property set to `true` as well as access to the host as an unprivileged user. The most common case for this would be systems using `incus-user` with the less privileged `incus` group to provide unprivileged users with an isolated restricted access to Incus. Such users may be able to create a custom storage volume with the necessary property (depending on kernel and filesystem support) and can then write a setuid binary from within the container which can be executed as an unprivileged user on the host to gain root privileges. A patch for this issue is expected in versions 6.0.6 and 6.19.0. As a workaround, permissions can be manually restricted until a patched version of Incus is deployed.