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.
Specifying additional fields will cause the parser to fail.

For example:
`printf "username:pat" | openssl base64 -A`

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

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.

Signed-off-by: Alano Terblanche <[email protected]>