Update parent image glossary definition (#9617)
Update parent image glossary definition (#9617)

All Dockerfiles must have a FROM statement (as confirmed in the [builder documentation](https://docs.docker.com/engine/reference/builder/#from)). This change clarifies the phrasing within parent image. See [this stackoverflow post](https://stackoverflow.com/q/58330046/596285) for how the current phrasing has confused others.
thaJeztah/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io