Use ISRG_ROOT_X1 as DST_ROOT is not available

core/nginx/conf/tls.conf

@@ -3,6 +3,7 @@ ssl_certificate_key {{ TLS[1] }};
 {% if TLS_FLAVOR in ['letsencrypt','mail-letsencrypt'] %}
 ssl_certificate {{ TLS[2] }};
 ssl_certificate_key {{ TLS[3] }};
+ssl_trusted_certificate /etc/ssl/certs/ca-cert-ISRG_Root_X1.pem
 {% endif %}
 ssl_session_timeout 1d;
 ssl_session_tickets off;