This is a bit of a noob question, but what what do you type in after getting this message?
# Executing docker install script, commit: 490beaa
Warning: the "docker" command appears to already exist on this system.
If you already have Docker installed, this script can cause trouble, which is
why we're displaying this warning and provide the opportunity to cancel the
installation.
If you installed the current Docker package using this script and are using it
again to update Docker, you can safely ignore this message.
You may press Ctrl+C now to abort this script.
+ sleep 20
+ sh -c apt-get update -qq >/dev/null
Just copy/paste the “sh -c apt-get update -qq >/dev/null” part?