cli/command/container: --use-api-socket: don't write empty credentials

[thaJeztah]

• follow-up to run: flag to include the docker socket #5858

Before this patch, a valid, but empty set of credentials would still
write a config-file to the container and set DOCKER_CONFIG:

mkdir -p tmpConfig
export DOCKER_CONFIG=$PWD/tmpConfig

echo '{}' > "${DOCKER_CONFIG}/config.json"
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {}
}

echo '{"auths": {}}' > "${DOCKER_CONFIG}/config.json"
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
{
	"auths": {}
}

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"
		}
	}
}

With this patch, the DOCKER_CONFIG env-var and config-file are only created
if we have credentials to set;

mkdir -p tmpConfig
export DOCKER_CONFIG=$PWD/tmpConfig

echo '{}' > "${DOCKER_CONFIG}/config.json"
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
cat: can't open '/run/secrets/docker/config.json': No such file or directory

echo '{"auths": {}}' > "${DOCKER_CONFIG}/config.json"
docker run --rm --use-api-socket alpine cat /run/secrets/docker/config.json
cat: can't open '/run/secrets/docker/config.json': No such file or directory

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"
		}
	}
}

- Human readable description for the release notes

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 58.89%. Comparing base (79ab3cb) to head (711fcae).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6018      +/-   ##
==========================================
- Coverage   58.89%   58.89%   -0.01%     
==========================================
  Files         358      358              
  Lines       29961    29962       +1     
==========================================
  Hits        17647    17647              
- Misses      11333    11334       +1     
  Partials      981      981              

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[thaJeztah]

@Benehiko PTAL 🤗