It would appear that perhaps this is not possible to do using Caddy. I found a post here Snap caddy https that seemed to imply that one way to make this work is to install and run nginx instead. I am not quite sure how to do that when rocketchat is installed using snaps. Perhaps someone can let me know if this is possble, and perhaps a pointer as to what is managing the web site in the snap so I can redirect it to nginx.