Add warning regarding config upgrade in docker.md
Add warning regarding config upgrade in docker.md

Green-Star/PeerTube,Chocobozzz/PeerTube,Green-Star/PeerTube,Green-Star/PeerTube,Chocobozzz/PeerTube,Chocobozzz/PeerTube,Chocobozzz/PeerTube,Green-Star/PeerTube