Add `-y` option flag to `add-apt-repository`

Due to the behaviour change, need to add `-y` option for
`add-apt-repository` command to operate without human interactions.

cookbooks/docker/install.rb

@@ -11,7 +11,7 @@ end
 execute 'add-apt-repository \
    "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
    $(lsb_release -cs) \
-   stable"' do
+   stable" -y' do
   not_if 'which docker'
 end