dockerignore-wildcard-bug

Demonstrates a bug in dockerignore wildcards (**)

Repro steps:

Checkout this directory and run make all

Expected result:

include.txt should be in the docker context.

exclude.txt should be ignored.

Actual result:

Both are included in the docker context.

Other notes

The underlying bug is this code:

https://github.com/moby/moby/blob/v19.03.12/pkg/fileutils/fileutils.go#L83

which assumes that the number of directory separators in the pattern can be used to determine the "directory" length, which is not a correct assumption when wildcards are present.

Interestingly, the exclude pattern is necessary to reproduce this problem. That's because this code:

https://github.com/moby/moby/blob/v19.03.12/pkg/archive/archive.go#L843

skips directories purely as an optimization. Adding an exclusion pattern (!a/b/c/include.txt) makes this code go down the "slow" codepath.