Feature: Enable multi-arch Docker images

[stumpylog]

Draft until #6 is decided and I can rebase or drop the extra commit.

• Use docker/metadata-action to handle getting image tags and manage :latest. For a tag x.y.z, this will produce the image tags :x.y.z, :x.y and :x, as well as updating :latest as necessary
  • Also provides standard image labels for free
  • See the output here
• Use docker/login-action to handle authenticating to Docker Hub.
  • I couldn't fully test that portion, for obvious reason, but it should work identically
• Uses docker/build-push-action and associated
  • adds support for amd64, arm64 and armv7
  • handles the image tags and labels from the metadata
  • Could support caching if desired
• Some initial logic that could allow pull requests to build, but not push the image

[BaldissaraMatheus]

Thanks for your contribution @stumpylog, #6 is already merged sp you can rebase and set it as ready for review. This PR will also resolve #8

[stumpylog]

Done! Let me know if you've questions, I know it's a pretty large changeset.