You can still get Let’s Encrypt certificates if you use the DNS challenge with your DNS provider, if you have a domain name to use.
If you don’t have a domain name, it’s a longer story. You can use tls internal, but that requires knowing what you’re doing.
Is WebTransport server on the same VM? Is it in a container or a VM (you used both words)? Are you accessing Caddy from a different machine? You have to share more details