Docker build output tar file not working

Description

If you want to build a tar file of an image with docker build --output type=tar,dest=out.tar . like described here: https://docs.docker.com/engine/reference/commandline/build/#custom-build-outputs it just does not work. there is no file created! Also the man docker-build doesn't inclode information about the --output flag.

Steps to reproduce the issue:

1. run docker build --output type=tar,dest=out.tar . in a directory with a Dockerfile

Describe the results you received:
out.tar is not created

Describe the results you expected:
out.tar is created

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.12
 API version:       1.40
 Go version:        go1.13.10
 Git commit:        48a66213fe
 Built:             Mon Jun 22 15:45:50 2020
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.12
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.13.10
  Git commit:       48a66213fe
  Built:            Mon Jun 22 15:44:21 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 2
  Running: 2
  Paused: 0
  Stopped: 0
 Images: 28
 Server Version: 19.03.12
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 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
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.7.0-2-amd64
 Operating System: Debian GNU/Linux bullseye/sid
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.779GiB
 Name: work
 ID: EE5A:L6DH:PNMV:D7H4:BKJD:MBNN:KF4T:F5UW:PHZY:KCDZ:3GWH:QFUR
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: true
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No swap limit support

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

Start with the docker build --output type=tar,dest=out.tar . entry point and the man docker-build documentation mentioned in the report. Reproduce the command with a Dockerfile, then trace why out.tar is not created; done means the tar file is created and the --output flag is documented in man docker-build.