dockerfile2llb: ADD wildcard tar doesn't extract tarball to destination

[cirocosta]

Hey,

First of all, thanks a lot for all of your efforts on buildkit!

Onto the issue,

It appears that builds using buildkit differ a bit from those using the plain old docker when it comes to the ADD directive when combined with wildcards.

For instance, while ADD ./*.tgz /dest in the old docker builder we end up with the extracted contents under /dest, with buildkit, that's not the observed.

Here's a reproducible:

tar --list --file=./mything.tgz
./a
./b
./c

# vim: set ft=Dockerfile:
# Dockerfile.with-wildcard
FROM busybox

ADD ./*.tgz /dest

# vim: set ft=Dockerfile:
# Dockerfile.without-wildcard
FROM busybox

ADD ./mything.tgz /dest

By building them with DOCKER_BUILDKIT enabled and disabled, we can observe how different the results are.

Here's a Makefile that can make that process simpler:

run: | build-with-wildcard build-without-wildcard run-with-wildcard run-without-wildcard

run-%:
	docker run \
	  --rm \
	  $* \
	  /bin/sh -c 'ls /dest'

build-%:
	  docker build \
	    --no-cache \
	    -f ./Dockerfile.$* \
	    -t $* \
	    .

Executing make run and DOCKER_BUILDKIT=1 make run:

DOCKER_BUILDKIT=1 make run
# ...
docker run \
	  --rm \
	  with-wildcard \
	  /bin/sh -c 'ls /dest'
/dest
docker run \
	  --rm \
	  without-wildcard \
	  /bin/sh -c 'ls /dest'
a
b
c

make run
docker run \
	  --rm \
	  with-wildcard \
	  /bin/sh -c 'ls /dest'
a
b
c
docker run \
	  --rm \
	  without-wildcard \
	  /bin/sh -c 'ls /dest'
a
b
c

Something I noticed is that there are no tests for this case: https://github.com/tonistiigi/buildkit/blob/6cb4256e81dd20691d9c71a988d0791aee02e00b/frontend/dockerfile/dockerfile_test.go#L38-L49

Please let me know if you'd like any extra info.

Thanks!