Added adding the gpg key

docs/sources/installation/ubuntulinux.rst

@@ -66,6 +66,9 @@ to follow them again.*
 
 .. code-block:: bash
 
+   # Add the Docker repository key to your local keychain
+   sudo sh -c "curl http://get.docker.io/gpg | apt-key add -"
+
    # Add the Docker repository to your apt sources list.
    sudo sh -c "echo deb https://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list"
 
@@ -116,6 +119,9 @@ to follow them again.*
 
 .. code-block:: bash
 
+   # Add the Docker repository key to your local keychain
+   sudo sh -c "curl http://get.docker.io/gpg | apt-key add -"
+
    # Add the Docker repository to your apt sources list.
    sudo sh -c "echo deb https://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list"