Fix ACME HTTP-01 passthrough in HAProxy config

Add an ACL that routes /.well-known/acme-challenge/ requests on :80
to Caddy instead of redirecting to HTTPS, so Let's Encrypt certificate
issuance works out of the box.

Also simplify Caddyfile to use Caddy's http_port/https_port directives.

contrib/sni-router/docker-compose.yml

@@ -44,6 +44,7 @@ services:
       - caddy_data:/data
       - ./www:/srv:ro
     expose:
+      - "80"
       - "8443"
     environment:
       DOMAIN: ${DOMAIN:-example.com}