Laundry list of smells from this resource

This resource hasn't aged well. We've long suspected a new approach may be necessary, potentially including a complete rewrite and/or breaking it up into separate resource types, each focused on a specific workflow.

I'm going to use this issue as a dumping ground of sorts for now, in hopes that it eventually leads to a more focused direction that can skewer a few of these piranhas at once.

1. There are too many knobs, and way too many ways to put.
2. The testing situation is improving, but pretty bad.
3. Important parts are written in bash and are subject to the docker CLI output format changing. Some things are already broken because of this.
4. There are too many ways to configure credentials.
5. There's no caching by default, and it's unclear if everyone would want that. cache: true even pulls down the image willy-nilly, preventing Concourse caching of the image (ironically). (see https://github.com/concourse/docker-image-resource/issues/121, https://github.com/concourse/docker-image-resource/issues/148, https://github.com/concourse/docker-image-resource/pull/188)
   • Caching all the time is great and all until it's not. Lines like apt-get update && apt-get -y install foo will need the cache busted in order to get a new foo.
6. There isn't a great way to integrate Dockerfiles with Concourse-fetched dependencies while also having the image easy to build locally during development (you'd be missing all the dependencies).
7. There's no way to build without pushing to a registry. To be honest, as a resource that makes sense (there must be an external source of truth), but a reusable task would be great for this.
8. It embeds a Docker daemon. Would it be easier to point at an external one? What impact might this have on caching? Would this be an acceptable assumption for most people? How would the resource be pointed at the daemon?

The issue names no files or tests; start by reviewing the docker-image-resource implementation and the linked issues #121, #148, and pull request #188. Separate the listed concerns into a focused scope, with an agreed direction and a separately actionable issue as the definition of done.