Fixes issue

Describe the change

This directly injects the docker config credentials using an environment variable instead of a config file. See docker/cli#6008 for the upstream project change.

How to verify it

docker run -e DOCKER_AUTH_CONFIG="$(cat ~/.docker/config.json)" regclient/regctl manifest get $image -v trace

Note the registries processed in the debug logs.

Changelog text

• Feat: Support DOCKER_AUTH_CONFIG variable.

Please verify and check that the pull request fulfills the following requirements

Documentation additions could be added to the regclient.org site in a separate PR.

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed