dockerignore: Exclamation mark with the pattern ** isn't including all sub directories's files but only the top level one

[roi-meir]

Description
I the dockerignore file exclamation mark with the pattern ** isn't including all sub directories's files but only the top level one.
This issue prevent building a dockerignore file that allow coping of only specific extensions.
I'm not sure if it is the intended behaviour or not but I didn't see it documented anywhere.

Steps to reproduce the issue:

1. Create the following directory structure:

top_level_python_file.py
top_level_txt_file.txt
subdir/subdir_python_file.py

2. Create a .dockerignore with the following lines

*
!**/*.py

3. Run docker build with COPY . .

Describe the results you received:
only top_level_python_file.py is copied and not subdir_python_file.py

Describe the results you expected:
I expected all the files ending with .py to be copied.

Additional information you deem important (e.g. issue happens only occasionally):
The same happens where the pattern is !subdir/**/*.py. In such case the file subdir/another_subdir/subdir_python_file.py also won't be copied.

I think it happens because the code in archive.go that check for prefix of the pattern and not use the pattern regex

  	dirSlash := relFilePath + string(filepath.Separator)
	for _, pat := range pm.Patterns() {
		if !pat.Exclusion() {
			continue
		}
		if strings.HasPrefix(pat.String()+string(filepath.Separator), dirSlash) {
			// found a match - so can't skip this dir
			return nil
		}
	}

I will be happy to suggest a patch if you think this issue is indeed a bug and not the intended behaviour.

Output of docker version:

Client: Docker Engine - Community
 Cloud integration: 1.0.4
 Version:           20.10.2
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        2291f61
 Built:             Mon Dec 28 16:12:42 2020
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.2
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       8891c58
  Built:            Mon Dec 28 16:15:23 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.4.3
  GitCommit:        269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  scan: Docker Scan (Docker Inc., v0.5.0)

Server:
 Containers: 66
  Running: 0
  Paused: 0
  Stopped: 66
 Images: 301
 Server Version: 20.10.2
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: 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: io.containerd.runtime.v1.linux runc io.containerd.runc.v2
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc version: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 4.19.121-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 1.942GiB
 Name: docker-desktop
 ID: 3E45:LCYJ:JIUZ:T63O:PEUY:LQFM:N6HQ:UVEX:4DYI:RSGO:ZUR3:KVBX
 Docker Root Dir: /var/lib/docker
 Debug Mode: true
  File Descriptors: 41
  Goroutines: 46
  System Time: 2021-01-07T22:18:06.382286871Z
  EventsListeners: 3
 HTTP Proxy: gateway.docker.internal:3128
 HTTPS Proxy: gateway.docker.internal:3129
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false
 Product License: Community Engine

Additional environment details (AWS, VirtualBox, physical, etc.):

[roi-meir]

I see now that this issue is a duplicate of moby/moby#30018 so I'm closing it.