Build fails with "Directory /host/build/rpms must exist" after upgrading to Docker 23

[markusboehme]

Platform I'm building on: Fedora 36 on x86_64

What I expected to happen: The build of individual packages and entire images to succeed after upgrading from Docker 20 to Docker 23.

What actually happened: The build failed:

[...]
   > [rpmbuild 6/7] RUN --mount=target=/host     ln -s /host/build/rpms/*.rpm ./rpmbuild/RPMS     && createrepo_c         -o ./rpmbuild/RPMS         -x '*-debuginfo-*.rpm'         -x '*-debugsource-*.rpm'         --no-database         /host/build/rpms     && cp .rpmmacros /etc/rpm/macros     && dnf -y         --disablerepo '*'         --repofrompath repo,./rpmbuild/RPMS         --enablerepo 'repo'         --nogpgcheck         builddep rpmbuild/SPECS/grub.spec:
  #17 0.213 Directory /host/build/rpms/ must exist
[...]

How to reproduce the problem:

1. Run Docker 23
2. cargo make clean
3. cargo make -e BUILDSYS_VARIANT=metal-dev

[markusboehme]

I played around with this a bit and found a change in the handling of .dockerignore files between Docker 20 and Docker 23 must be causing this error. The following setup can serve as a small reproducer:

mkdir dockerignore-test && cd $_
mkdir -p dir/subdir
cat >.dockerignore <<EOF
/dir/*
!/dir/subdir/*
EOF
cat >Dockerfile <<EOF
FROM alpine:latest
RUN --mount=target=/context test -d /context/dir/subdir
EOF

Building with DOCKER_BUILDKIT=1 docker build . succeeds on Docker 20, but fails on Docker 23. Docker 23 will only include the subdir if it's not empty. Docker 20 will include it regardless.

There is a similar situation with the build/rpms directory in Bottlerocket's .dockerignore file when starting with a clean working directory. I'll prepare a PR.

[markusboehme]

Not opening a GitHub issue in the Moby repo for this. The new behavior seems reasonable to me. From looking around, changes in how .dockerignore is interpreted also don't seem all that uncommon.