Rethink `unused-exporter` docker image workflow

[logyball]

As part of PR #79, we added a ton of improvements, including building a docker image for the unused-exporter. With that done, we need to now make it better!

### Tasks
- [ ] Push to a publicly available repo
- [ ] Push an image for `unused` and `unused-exporter`?  If so, rename the docker images appropriately
- [ ] Automatically build/push from CI - complete with authorization workflow, etc

[Pokom]

I believe there are shared workflows that you can use. See how we do it within cloudcost-exporter: https://github.com/grafana/cloudcost-exporter/blob/main/.github/workflows/docker.yml#L30-L37.

Here's a link to the action directly with associated documentation: https://github.com/grafana/shared-workflows/tree/main/actions/build-push-to-dockerhub

@the-it set this up, so he can speak more to what was needed to get the credentials for the repo setup.