Document how to add additional labels to a Docker image

 * Add documentation on how to add labels to a Docker image (either in `createDockerFile` or in `buildDockerImage`)
 * It appears that in version 5.0.x, it was possible to access `labels` on the `createDockerFile` task, and this is no longer supported in version 5.2.x. This was never public API, but we should document in the migration guide to use `label()` instead.