Pass --all when building manifests

[stuartwdouglas]

This means the task can also take manifest lists as an input, which is useful when dealing with OCI artifacts.

[chmeliik]

Could you explain why we want this?

[ralphbean]

Checking the help for buildah manifest add:

      --all                           add all of the list's images if the image is a list

I suppose then that this helps when making an image index of image indices.

And, although I wasn't a part of the development of this, I can imagine what it's for. For parts of the instructlab project, they want to build disk images with #1059, where (I'm guessing) each build task produces an image index, which then needs to be concatenated onto a longer image index here.

[ralphbean]

Thanks!