Properly install ppa, avoid GPG key warning

Upstream-commit: 4e576f047fc8a5a75fa88a66132bdade4f3a1e44
Component: engine

components/engine/docs/sources/installation/ubuntulinux.rst

@@ -54,9 +54,9 @@ which makes installing Docker on Ubuntu very easy.
 .. code-block:: bash
 
    # Add the PPA sources to your apt sources list.
-   sudo sh -c "echo 'deb http://ppa.launchpad.net/dotcloud/lxc-docker/ubuntu precise main' > /etc/apt/sources.list.d/lxc-docker.list"
+   sudo apt-get install python-software-properties && sudo add-apt-repository ppa:dotcloud/lxc-docker
 
-   # Update your sources, you will see a warning.
+   # Update your sources
    sudo apt-get update
 
    # Install, you will see another warning that the package cannot be authenticated. Confirm install.