fix: env SMTP_PORT is empty caused err when launching (#5742)

docker/.env.example

@@ -405,7 +405,7 @@ RESEND_API_KEY=your-resend-api-key
 
 # SMTP server configuration, used when MAIL_TYPE is `smtp`
 SMTP_SERVER=
-SMTP_PORT=
+SMTP_PORT=465
 SMTP_USERNAME=
 SMTP_PASSWORD=
 SMTP_USE_TLS=true