Add restart policy for Caddy (eg start Caddy when docker daemon boots/restarts)

[davidjb]

The Caddy container currently has no restart policy, meaning that if the host/Docker daemon restarts (or Caddy errors/quits for some reason), Caddy will be left offline and thus Cabot becomes inaccessible, requiring a manual start or restart to get Caddy online. The other containers (https://github.com/cabotapp/docker-cabot/blob/master/docker-compose.yml) have restart policies of always; this PR adds the same for Caddy.