sysutils/buildah: new port

The buildah package provides a command line tool which can be used to

• create a working container from scratch

or

• create a working container from an image as a starting point
• mount/umount a working container's root file system for manipulation
• save container's root file system layer to create a new image
• delete a working container or an image.

PR: 267184
Reviewed by: arrowd
Reviewed by: dch
Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D37324