add `copy` equivalent from containerfile

[cgwalters]

Describe the bug

Right now we have add-files but it's unergonomic to use with a whole filesystem tree. Let's add something more like COPY in a containerfile.

This would allow taking over the role of the ostree-layers and ostree-override-layers.

[jlebon]

That's #1645, right?

[cgwalters]

Yep