Document running local services when using Earthly

[binkley]

The previous containerized build tool, Batect, included direct support for starting the container with a long-running server making it easy to hit endpoints and debug.

The current project is nor for a service, but a "Hello, world" cmd line.
Tackling this card involves standing something up that is long-running so you can hit endpoints.

Earthly is focused on build, and does not have direct support for running your project locally as a server for you to hit endpoints with.

• Research best practice with Earthly
• Update documentation with Docker cmd line examples
• Update the example project with a long-running REST service
• Provide a local cmd line script that runs the service, and hits the endpoints to demonstrate

Refs

• https://docs.earthly.dev/basics/part-1-a-simple-earthfile -- this and the docs for Earthly show it isn't for running a service, but only for building
• https://github.com/binkley/kotlin-spring-boot-hateoas-database -- has an example shell script for demoing a local service