feat: Support for Amazon ECR Public Gallery

[peterbuga]

Is your feature request related to a problem? Please describe.
Not a dealbreaker for now but it would be nice to support Amazon ECR Public Gallery as a registry type.

example repository that it's quite known and they moved the images to ECR https://gallery.ecr.aws/gravitational/teleport-distroless

Describe the solution you'd like
to be able to specify in config

    latest_version:
      type: "github"
      url: "gravitational/teleport"
      require:
        docker:
          type: "ecr"
          image: "gravitational/teleport-distroless"
          tag: "{{ version }}"

Describe alternatives you've considered
Currently not checking if docker images are available for the specific release.

Additional context
-

[JosephKav]

Hi,
I'd never heard of ECR Public Gallery until this issue. I can take a look at adding support for it, but do want to get a few other features added first.
Thanks for raising the suggestion though, always nice to hear about new uses for the app :)