The "lilypond" image is intended for running LilyPond after it has
been built.
Supporting changes to docker-compose.yaml and documentation will
follow. The use of the "lilypond-build" container is unchanged as
yet.
2.
Docker containers: set a hostname and use a two-line prompt
1. Dockerfile: define a "lilypond" image
2. Docker containers: set a hostname and use a two-line prompt