chapter9:optional: add --rm to docker run busybox echo hello world

This commit proposes to replace this command

  $ docker run busybox echo hello world

with

  $ docker run --rm busybox echo hello world

so that docker deletes the container when the container stops.

In this chapter, the docker commands never restart a stopped
container. Because of that, each `docker run` command could
use `--rm` to remove the container when the container stops.

Without `--rm`, the stopped containers accumulate.
`docker ps -a` shows the stopped containers not yet deleted.

Author: zipperer

chapter_09_docker.asciidoc

@@ -496,7 +496,7 @@ Follow those, and then test your installation by running:
 
 [subs="specialcharacters,macros"]
 ----
-$ pass:quotes[*docker run busybox echo hello world*]
+$ pass:quotes[*docker run --rm busybox echo hello world*]
 Unable to find image 'busybox:latest' locally
 [...]
 latest: Pulling from library/busybox
@@ -512,6 +512,7 @@ What's happened there is that Docker has:
 * Downloaded the image from DockerHub
 * Created a container based on that image
 * Started up that container, telling it to run `echo hello world`
+* Deleted that container when the container stopped
 * And we can see it worked!
 
 Cool! We'll find out more about all of these steps as the chapter progresses.