Use DOCKER_AUTH_CONFIG env as credential store

[Benehiko]

This patch enables the CLI to natively pick up the DOCKER_AUTH_CONFIG
environment variable and use it as a credential store.

The DOCKER_AUTH_CONFIG value should be a JSON object and must store
the credentials in a base64 encoded string under the auth key.

Credentials stored in DOCKER_AUTH_CONFIG would take precedence over any
credential stored in the file store (~/.docker/config.json) or native store
(credential helper).

Destructive actions, such as deleting a credential would result in a noop if
found in the environment credential. Credentials found in the file or
native store would get removed.

- What I did

- How I did it

- How to verify it

printf "username:pat" | openssl base64 -A

Setup the DOCKER_AUTH_CONFIG environment variable

DOCKER_AUTH_CONFIG='{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "aGk6KTpkY2tyX3BhdF9oZWxsbw=="
    }
  }
}'

docker buildx bake --set binary.platform=linux/arm64

docker run -it -v ./build/docker-linux-arm64:/bin/docker -v /var/run/docker.sock:/var/run/docker.sock --env DOCKER_AUTH_CONFIG alpine:latest /bin/ash

/ # docker login
...
Login Succeeded

- Human readable description for the release notes

Use `DOCKER_AUTH_CONFIG` as a credential store

- A picture of a cute animal (not mandatory but encouraged)

[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 32 lines in your changes missing coverage. Please review.

Project coverage is 55.56%. Comparing base (9e50654) to head (9b83d5b).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6008      +/-   ##
==========================================
+ Coverage   55.03%   55.56%   +0.52%     
==========================================
  Files         361      356       -5     
  Lines       30153    29999     -154     
==========================================
+ Hits        16596    16670      +74     
+ Misses      12599    12366     -233     
- Partials      958      963       +5     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[thaJeztah]

• quick draw up; simplify unmarshaling, and other nits Benehiko/docker-cli#2

[vvoland]

I noticed I had some pending comments

[thaJeztah]

LGTM

[thaJeztah]

@vvoland PTAL

[vvoland]

Overall looks good, but left some comments

[vvoland]

LGTM, thanks!