kind-gcr.sh example does not work for authentication token

[MaxRenaud]

What happened:
The example on https://kind.sigs.k8s.io/docs/user/private-registries/#use-an-access-token does not work since Docker 18+ will use the OS Credential Store by default.
See https://docs.docker.com/engine/reference/commandline/login/#credentials-store
The generated client.json file will not contain the authentication token:

% gcloud auth print-access-token | \
  docker login -u oauth2accesstoken --password-stdin https://gcr.io
% cat ${DOCKER_CONFIG}/config.json
{
	"auths": {
		"gcr.io": {}
	},
	"HttpHeaders": {
		"User-Agent": "Docker-Client/19.03.12 (darwin)"
	},
	"credsStore": "osxkeychain"
}%

What you expected to happen:
I expected the config.json file to contain an "auth" key under auths["gcr.io"]

How to reproduce it (as minimally and precisely as possible):

% export DOCKER_CONFIG=$(mktemp -d)
% rm ${DOCKER_CONFIG}/config.json
% gcloud auth print-access-token | \
  docker login -u oauth2accesstoken --password-stdin https://gcr.io
% cat ${DOCKER_CONFIG}/config.json

Anything else we need to know?:
PR #1825

Environment:

• kind version: (use kind version): kind v0.8.1 go1.14.2 darwin/amd64
• Kubernetes version: (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.0", GitCommit:"9e991415386e4cf155a24b1da15becaa390438d8", GitTreeState:"clean", BuildDate:"2020-03-25T14:58:59Z", GoVersion:"go1.13.8", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.2", GitCommit:"52c56ce7a8272c798dbc29846288d7cd9fbae032", GitTreeState:"clean", BuildDate:"2020-04-30T20:19:45Z", GoVersion:"go1.13.9", Compiler:"gc", Platform:"linux/amd64"}

• Docker version: (use docker info):

Client: Docker Engine - Community
 Version:           19.03.12
 API version:       1.40
 Go version:        go1.13.10
 Git commit:        48a66213fe
 Built:             Mon Jun 22 15:41:33 2020
 OS/Arch:           darwin/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 15:49:27 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

• OS (e.g. from /etc/os-release): Darwin Kernel Version 19.5.0

[BenTheElder]

fixed in #1825, thanks!