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/arm64docker 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 

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[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!