use env vars for subdomain and domain in caddy

docker-compose.caddy.yml

@@ -4,6 +4,8 @@ services:
             context: /opt/caddy/
         container_name: caddy
         environment:
+        - DOMAIN=veenboer.xyz
+        - SUBDOMAIN=rik
         - AWS_REGION=eu-west-1
         - AWS_ACCESS_KEY_ID=${AWS_ACCESS_KEY_ID:?}
         - AWS_SECRET_ACCESS_KEY=${AWS_SECRET_ACCESS_KEY:?}