Download Ubuntu 13.04 repository key over HTTPS

docs/sources/installation/ubuntulinux.rst

@@ -129,7 +129,7 @@ to follow them again.*
 
    # Add the Docker repository key to your local keychain
    # using apt-key finger you can check the fingerprint matches 36A1 D786 9245 C895 0F96 6E92 D857 6A8B A88D 21E9
-   sudo sh -c "curl http://get.docker.io/gpg | apt-key add -"
+   sudo sh -c "curl https://get.docker.io/gpg | apt-key add -"
 
    # Add the Docker repository to your apt sources list.
    sudo sh -c "echo deb http://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list"