`swift-container-plugin`'s use of local registry instances is unclear

#134 points out that there is no documentation on how to set up a local registry. In addition, the reason the documentation and demos use a local registry is not explained.

`containertool` is currently only able to push an image to a registry - it cannot push to the local container runtime's image store. If pushing to a public registry or private network registry is not suitable, a local instance of [`registry:2`](https://hub.docker.com/_/registry) can be used instead. The documentation should make this clear.

Gaining the ability to push directly to a local engine's image store (#68), or save to disk, would provide alternatives to using a local registry.

Review the documentation and demos referenced by the issue, then read #134 for the missing local-registry context. Explain that containertool currently pushes only to registries, document how to set up a local registry:2 instance, and state why the examples use one. Done when the setup and rationale are clear and the alternatives are identified.

Written by the indexing model from the issue text.