clarify what a `.wh.` file does

[jeffective]

Please clarify the behavior of a whiteout file being applied to an empty dirname and basename.

For example, if I create a layer with the following content:

.wh.
etc/config.txt

Is this the expected behavior?

• wipes all content from previous layers
• adds file etc/config.txt

[sudo-bmitch]

I don't believe the behavior of a .wh. without a file or directory appended to the name is defined. To define that behavior in the spec, I'd want to compare the behavior of existing runtimes.

To delete all of the previous layer's contents from the image being built, that should be ./.wh..wh..opq, but I'm not sure why you would do that vs starting from a new image without any layers.

[cyphar]

Hmmm, it seems that umoci will treat .wh. like an opaque whiteout. We should probably be erroring out in this case.