buildah-add man page
buildah add — Add the contents of a file, URL, or a directory to a container.
buildah add containerID SRC [[...] DEST]
Adds the contents of a file, URL, or a directory to a container's working directory or a specified location in the container. If a local source file appears to be an archive, its contents are extracted and added instead of the archive file itself. If a local directory is specified as a source, its contents are copied to the destination.
buildah add containerID '/myapp/app.conf' '/myapp/app.conf'
buildah add containerID '/home/myuser/myproject.go'
buildah add containerID '/home/myuser/myfiles.tar' '/tmp'
buildah add containerID '/tmp/workingdir' '/tmp/workingdir'
buildah add containerID ' ⟨https://github.com/projectatomic/buildah/blob/master/README.md'⟩ '/tmp'
buildah add containerID 'passwd' 'certs.d' /etc