Enable auth against multiple registries

[PhilipBehrenberg]

Issue

If a user needs to push the resulting image to a private registry they have to give the private registry's credentials, via username and password. This means that every build image will be pulled from DockerHub anonymously. This causes problems on a larger team where you can hit DockerHub's pull rate limit (100pulls/6hours).

Proposal

Allow the user to, alternatively to user/pass, supply a imagePullSecret (like you can with drone) to the plugins/docker container. This would allow a user to give credentials for any number of registries, public and private. Pulling from DockerHub with AccountA and pushing to PrivateReg with AccountB.

[bradrydzewski]

👋 hey there, this is supported by using the config setting like this:

- name: build
  image: plugins/docker
  settings:
    repo: foo
    tags: latest
    config: |
      {
          "auths": {
              "https://index.docker.io/v1/": {
                  "auth": "c3R...zE2"
              }
          }
      }

or with a secret like this:

- name: build
  image: plugins/docker
  settings:
    repo: foo
    tags: latest
    config:
      from_secret: bar

you can see a few other options / examples in this thread (see the Docker-in-Docker Plugins section)
https://discourse.drone.io/t/how-to-prevent-dockerhub-pull-rate-limit-errors/8324