[BUG] Docker ignores the credentials stored in /root/.docker/config.json

Description
In salt 3000.3 docker ignores the credentials stored in /root/.docker/config.json and fails authenticating agains the private registry.

I'm using a masterless setup with salt 3000.3 without any docker registry defined in pillars, instead of that I have created the docker config file /root/.docker/config.json with the auth configuration from salt:

/root/.docker/config.json:
  file.managed:
    - source: salt://docker/templates/docker-config.json.jinja
    - template: jinja
    - user: root
    - group: root
    - makedirs: True
    - mode: '0600'

This file contains a valid authentication credentials for artifactory.

Then when trying to pull an image I get the error unknown: Authentication is required

I've also tried to do a docker.login but got the same error

authenticate-docker:
  module.run:
    - name: docker.login
    - require:
      - file: /root/.docker/config.json

From the documentation I assume that Salt version 3000 should be able to authenticate docker using the config file instead of pillars.

When login to the machine I'm able to pull the image, I guess the user I'm using is correctly using the config file for authentication.

Expected behavior
Docker to use the credentials stored in /root/.docker/config.json

Versions Report
Salt 3000.3

Start with the authentication section of salt.modules.dockermod and the docker.login entry point, then reproduce the masterless Salt 3000.3 setup using /root/.docker/config.json. Check how the pull and login paths locate and consume that file; done means private-registry image pulls authenticate successfully without pillar registry credentials.