Tag the latest docker images as `latest` on GH

[pixlwave]

Hey, first off thanks for Danger Swift - it's made such a difference to our workflow 🙌

I just tried configuring our GitHub workflow to use the latest tag like so:

uses: docker://ghcr.io/danger/danger-swift-with-swiftlint:latest

But the result was the following error:

Pull down action image 'ghcr.io/danger/danger-swift-with-swiftlint:latest'
  /usr/bin/docker pull ghcr.io/danger/danger-swift-with-swiftlint:latest
  Error response from daemon: manifest unknown
  Warning: Docker pull failed with exit code 1, back off 4.691 seconds before retry.
  /usr/bin/docker pull ghcr.io/danger/danger-swift-with-swiftlint:latest
  Error response from daemon: manifest unknown
  Warning: Docker pull failed with exit code 1, back off 4.539 seconds before retry.
  /usr/bin/docker pull ghcr.io/danger/danger-swift-with-swiftlint:latest
  Error response from daemon: manifest unknown
Error: Docker pull failed with exit code 1

I'm not sure if it's possible to tag the most recent docker images with latest as well as the version - the docs aren't particularly clear on this. But if it is possible, would it be something you would consider doing?