Give name to final stage in image

[leighmcculloch]

What

Name the final stage in the dockerfile.

Why

The image build process builds multiple independent parts at the same time and outputs the build logs with the name of each stage that is being built. All the stages except the last one have a name making it really easy to follow what's happening during the build. The final stage has no name and so its logs appear namespaced with "stage-18" which is not helpful. Naming the last stage will mean the logs will be annotated.