Add the --sign-by option to `buildah build-using-dockerfile`,
`buildah commit`, `buildah push`, and `buildah manifest push`. Add the
`--remove-signatures` option to `buildah pull`, `buildah push`, and
`buildah manifest push`. We just pass them to the image library, which
does all of the heavy lifting.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
Closes: #2085
Approved by: rhatdan