Consider installing as a Buildkite Plugin

[ngan]

Currently, BKB can be installed a gem or a docker image (using the docker plugin). A third, and maybe preferable, method of installation can be a bonafide Buildkite plugin. We’d essentially be copying what the docker plugin does but it might look cleaner:

steps:
  - label: “:toolbox”
    plugins:
      - Gusto/buildkite-builder#v1.2

The only downside I can see to this is duplicating all functionally of the docker plugin here. It would be great if we can depend on that plugin and delegate to it somehow.