Docker Login Command about docker-mixin

[carolynvs]

There are some neat things that I think we can plan for to take advantage of what bundles do. For example, the docker username and password will be passed into the bundle as credentials or parameters, and will be available to us as environment variables.

credentials:
- name: docker-user
  env: DOCKER_USERNAME
- name: docker-password
  env: DOCKER_PASSWORD

install:
  docker: login

The mixin can default the username and password fields to the environment variables DOCKER_USERNAME and DOCKER_PASSWORD so that people don't need to fill them in unless they are doing something different than the default.

Until we have that implemented with defaulting using environment variables, we can pass in credentials to the mixin like this (below). Porter will handle hidding the username and password from the output with ***'s

credentials:
- name: docker-user
  env: DOCKER_USERNAME
- name: docker-password
  env: DOCKER_PASSWORD

install:
  docker:
    login:
      username: "{{ bundle.credentials.docker-user }}"
      password: "{{ bundle.credentials.docker-password }}"