COPY multiple files with wildcard should fail when destination is a file

Description

Dockerfile reference for COPY states: If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

I have seen this work correctly in the past, but recently I noticed it is no longer failing as it should

Reproduce

echo test1 > test1.txt
echo test2 > test2.txt

Dockerfile:

FROM scratch

COPY *.txt test.txt

docker build .

Expected behavior

Docker build should fail because <src> (*.txt) resolves to multiple files but <dest> (test.txt) is not a directory. Instead, the build succeeds and a single file is copied (for me, it is test2.txt).

docker version

Client: Docker Engine - Community
 Version:           24.0.7
 API version:       1.43
 Go version:        go1.20.10
 Git commit:        afdd53b
 Built:             Thu Oct 26 09:08:17 2023
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          24.0.7
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.20.10
  Git commit:       311b9ff
  Built:            Thu Oct 26 09:08:17 2023
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.26
  GitCommit:        3dd1e886e55dd695541fdcd67420c2888645a495
 runc:
  Version:          1.1.10
  GitCommit:        v1.1.10-0-g18a0cb0
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    24.0.7
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.11.2
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.21.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 3
  Running: 0
  Paused: 0
  Stopped: 3
 Images: 3
 Server Version: 24.0.7
 Storage Driver: overlay2
  Backing Filesystem: btrfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: false
  userxattr: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc io.containerd.runc.v2
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 3dd1e886e55dd695541fdcd67420c2888645a495
 runc version: v1.1.10-0-g18a0cb0
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
 Kernel Version: 6.1.60-08594-g03a802b9a072
 Operating System: Debian GNU/Linux 11 (bullseye)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 6.485GiB
 Name: penguin
 ID: de47c6d0-1e57-42b6-9fce-995fa823c673
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled

Additional Info

No response

Start by running the Dockerfile reproduction with two matching files and confirm that COPY *.txt test.txt succeeds instead of failing. Trace BuildKit's Dockerfile COPY wildcard handling, then add coverage showing that multiple resolved sources require a directory destination and verify that the build fails with the expected error.