Refactor Swift image and include the gpg key

Testable locally with:

docker-pkg --info
-c dockerfiles/config.yaml
build --select="buster-swift53:" dockerfiles

The image is used by mediawiki/libs/metrics-platform: .pipeline/blubber/swift.yaml

version: v4
variants:
  test:
    base: docker-registry.wikimedia.org/dev/buster-swift53
    copies: [local]
    runs:
      insecurely: true
    entrypoint: [swift, test]