buildx is missing from docker:20-dind images

[anapsix]

Description

Docker 20-dind images are shipped with docker cli which seems to be missing buildx entirely.. which means support for buildx build --cache-from is not available.
Launching docker buildx build --help on my Macbook shows --cache-from available. However launching the same in container instance of docker:20-dind does not show --cache-from as valid option to buildx build.

Steps to reproduce the issue:

$ docker pull docker:20.10.6-dind
$ docker run -it --rm -e DOCKER_CLI_EXPERIMENTAL=enabled docker:20.10.6-dind sh -c 'docker --help | grep buildx && echo SUCCESS: buildx is available || echo ERROR: buildx is not available'
$ docker run -it --rm docker:20.10.6-dind sh -c 'docker --version; docker buildx build --help | grep cache-from || echo "ERROR: buildx build --cache-from is not supported"'

Describe the results you received:

ERROR: buildx is not available
Docker version 20.10.6, build 370c289
ERROR: buildx build --cache-from is not supported

Describe the results you expected:

SUCCESS: buildx is available
Docker version 20.10.6, build 370c289
      --cache-from stringArray   External cache sources (eg. user/app:cache, type=local,src=path/to/dir)

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

Output of docker version:
from docker:20.10.6-dind docker image

Client:
 Version:           20.10.6
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        370c289
 Built:             Fri Apr  9 22:42:10 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true
error during connect: Get http://docker:2375/v1.24/version: dial tcp: lookup docker on 192.168.65.5:53: no such host

Output of docker info:
from docker:20.10.6-dind docker image

Client:
 Context:    default
 Debug Mode: false

Server:
ERROR: error during connect: Get http://docker:2375/v1.24/info: dial tcp: lookup docker on 192.168.65.5:53: no such host
errors pretty printing info

Additional environment details (AWS, VirtualBox, physical, etc.):
Launching docker run -it --rm docker:20.10.6-dind sh -c ... on macOS 10.14.6 running Docker Desktop 3.3.1 (63152)

[ndeloof]

This issue should be moved to https://github.com/docker-library/docker

[anapsix]

created docker-library/docker#313

[anapsix]

For reference, docker-library/docker#156 includes discussion and clarification on why buildx plugin is not included in docker images. However, the PR (docker/docker-ce-packaging#415) is open to include base plugins in official docker images.