add --rm to docker run line

This would avoid leaving a (stopped) container lying around, as can be seen in `docker ps -a`.

I believe that, if you agree, multiple pages would want this change. Let me know and I'll push more commits (and I'm happy to squash afterwards).

Author: Habbie

content/engine/install/ubuntu.md

@@ -165,7 +165,7 @@ Docker from the repository.
    `hello-world` image.
 
    ```console
-   $ sudo docker run hello-world
+   $ sudo docker run --rm hello-world
    ```
 
    This command downloads a test image and runs it in a container. When the