feat: update docs to ensure devs always install from a real deb repo

docs/getting-started/install/debian.md

@@ -13,6 +13,7 @@ wget -nv -O - https://get.docker.com/ | sh
 # install dokku
 wget -nv -O - https://packagecloud.io/gpg.key | apt-key add -
 OS_ID="$(lsb_release -cs 2> /dev/null || echo "trusty")"
+echo "trusty utopic vivid wily xenial yakkety zesty artful bionic" | grep -q "$OS_ID" || OS_ID="trusty"
 echo "deb https://packagecloud.io/dokku/dokku/ubuntu/ ${OS_ID} main" | sudo tee /etc/apt/sources.list.d/dokku.list
 sudo apt-get update -qq > /dev/null
 sudo apt-get install -qq -y dokku