COPY --exclude does not exclude any files when source is not a directory

[codebling]

To reproduce, create two empty files, a and b, and a Dockerfile with the following contents:

# syntax=docker/dockerfile:1.7-labs
FROM scratch
COPY --exclude=a * .

The files a and b appear in the image.
This is contrary to the documentation, which states

to add all files starting with "hom", excluding files with a .txt extension:
COPY --exclude=*.txt hom* /mydir/

If the source is a directory, as in the slightly modified version of the Dockerfile above, a is not copied.

# syntax=docker/dockerfile:1.7-labs
FROM scratch
COPY --exclude=a . .

[codebling]

See also #4561

[tonistiigi]

cc @jsternberg , not sure if related to #4946

[codebling]

not sure if related to #4946

Based on this comment,

The reason why this seems to affect --exclude is because exclude is forcing the checksum to compute which paths it's allowed to follow and the broken symlink just happens to be included.

it looks like this issue is not related to that one, but I'm no expert here.