`containertool` should be able to apply labels to the images it produces

Add a `--label` multi-value command line parameter and add the value to the [`Labels`](https://github.com/apple/swift-container-plugin/blob/0c9ea97611f7344dc5408794f33df8c3c513e2a0/Sources/ContainerRegistry/Schema.swift#L124) field in the [`configuration inherited from the base image`](https://github.com/apple/swift-container-plugin/blob/0c9ea97611f7344dc5408794f33df8c3c513e2a0/Sources/containertool/containertool.swift#L184C60-L184C64).

Start in Sources/containertool/containertool.swift at the configuration inherited from the base image, then inspect the Labels field in Sources/ContainerRegistry/Schema.swift. Trace how command-line parameters are defined and passed into the image configuration. Done means containertool accepts multiple --label values and adds them to the produced image's Labels field.

Written by the indexing model from the issue text.