In this example, the `Dockerfile` defines three images. A base with two

children images.

`docker build` has a `--target` option that will allow you to select which

image to build in a `Dockerfile` like this. The value for `--target` is the

name given to the image which is the part after the `as`. In convey you can

set the `target` attribute to set this value.

The `convey.yml` will then build both child images and then run and remove

them.