Buildah: full container escape at build time

A flaw was found in Buildah (and subsequently Podman Build) which allows containers to mount arbitrary locations on the host filesystem into build containers. A malicious Containerfile can use a dummy image with a symbolic link to the root filesystem as a mount source and cause the mount operation to mount the host root filesystem inside the RUN step. The commands inside the RUN step will then have read-write access to the host filesystem, allowing for full container escape at build time.