cli/command/container: --use-api-socket: support DOCKER_AUTH_CONFIG

[thaJeztah]

• relates to run: flag to include the docker socket #5858
• relates to Use DOCKER_AUTH_CONFIG env as credential store #6008

cli/command/container: --use-api-socket: support DOCKER_AUTH_CONFIG

With this patch, the --use-api-socket flag can obtain credentials from
a validly formatted DOCKER_AUTH_CONFIG environment-variable. If the
env-var is not set, or doesn't contain credentials, it falls back to
attempting to read credentials from the CLI's configured credentials
store.

With this patch:

# Make sure there's no auth on disk first
mkdir -p tmpConfig
export DOCKER_CONFIG=$PWD/tmpConfig
rm -f $PWD/tmpConfig/config.json

# no credentials
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
cat: cant open '/run/secrets/docker/config.json': No such file or directory

# pass credentials through DOCKER_AUTH_CONFIG
DOCKER_AUTH_CONFIG='{"auths": {"https://index.docker.io/v1/": {"auth": "am9lam9lOmhlbGxv"}}}' docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {
		"https://index.docker.io/v1/": {
			"auth": "am9lam9lOmhlbGxv"
		}
	}
}

# credentials from file if no DOCKER_AUTH_CONFIG is set
echo '{"auths": {"https://index.docker.io/v1/": {"auth": "am9lam9lOmhlbGxv"}}}' > "${DOCKER_CONFIG}/config.json"
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {
		"https://index.docker.io/v1/": {
			"auth": "am9lam9lOmhlbGxv"
		}
	}
}

# same if DOCKER_AUTH_CONFIG is set, but doesn't contain credentials
DOCKER_AUTH_CONFIG='{}' docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {
		"https://index.docker.io/v1/": {
			"auth": "am9lam9lOmhlbGxv"
		}
	}
}

DOCKER_AUTH_CONFIG='{"auths": {}}' docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {
		"https://index.docker.io/v1/": {
			"auth": "am9lam9lOmhlbGxv"
		}
	}
}

- Human readable description for the release notes

Add support for `DOCKER_AUTH_CONFIG` for the experimental `--use-api-socket` option.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 18.51852% with 22 lines in your changes missing coverage. Please review.

Project coverage is 58.86%. Comparing base (4d8c241) to head (73be734).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6019      +/-   ##
==========================================
- Coverage   58.89%   58.86%   -0.04%     
==========================================
  Files         358      359       +1     
  Lines       29962    29980      +18     
==========================================
  Hits        17647    17647              
- Misses      11334    11352      +18     
  Partials      981      981              

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