build.sh

$ cat build.sh
container=$(buildah from alpine)                                                                                                                                                                      
mnt=$(buildah mount $container)                                                                                                                                                                         
set -ex; mkdir -p $mnt/work; touch $mnt/work/foo

container2=$(buildah from alpine)                                                                                                                                                                       
mnt2=$(buildah mount $container2)

set -ex;mkdir -p $mnt2/work
cp $mnt/work/foo $mnt2/work/

buildah umount $container2                                                                                                                                                                        
buildah commit $container2 test

buildah unshare , podman run

$ buildah unshare bash build.sh
$ podman run --rm test ls -l /work
total 0
-rw-rw-r--    1 root     root             0 Jun 11 07:59 foo

version

$ buildah version
Version:         1.21.0
Go Version:      go1.16.3
Image Spec:      1.0.1-dev
Runtime Spec:    1.0.2-dev
CNI Spec:        0.4.0
libcni Version:  
image Version:   5.12.0
Git Commit:      
Built:           Thu Jan  1 09:00:00 1970
OS/Arch:         linux/amd64

$ podman version
Version:      3.2.0
API Version:  3.2.0
Go Version:   go1.16.3
Built:        Wed Jun  9 23:24:16 2021
OS/Arch:      linux/amd64