Judge mailu's front by what mail needs, not only its web health

The image's own check asks nginx's /health and that dovecot runs, so the
front stayed healthy while mail could not flow (issue 688). It now also
needs port 25 to greet over SMTP and the served certificate to hold for
14 more days; certbot renews at 30, so a renewal stuck for two weeks is
said long before the certificate expires.

modules/mailu/module.json

@@ -560,7 +560,12 @@
       "name": "mailu-front",
       "image": "ghcr.io/mailu/nginx@sha256:36f98897cd1bc9d27628bbb4e04bdf60147af2ec7507d6da77f002c4f256896d",
       "health": {
-        "kind": "runtime"
+        "kind": "exec",
+        "command": "curl -m3 -skfLo /dev/null http://127.0.0.1:10204/health && kill -0 $(cat /run/dovecot/master.pid) && python3 -c \"import smtplib; s = smtplib.SMTP('127.0.0.1', 25, timeout=5); s.ehlo('health'); s.quit()\" && openssl x509 -checkend 1209600 -noout -in /certs/letsencrypt/live/mailu/nginx-chain.pem",
+        "interval": "60s",
+        "timeout": "15s",
+        "looks": 3,
+        "grace": "120s"
       },
       "network": "mailu",
       "env-file": [