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

[euanh]

#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 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.