Update SMTP environment variable name in docker-compose (#2444)

docker/docker-compose.yaml

@@ -108,7 +108,7 @@ services:
       MAIL_TYPE: ''
       # default send from email address, if not specified
       MAIL_DEFAULT_SEND_FROM: 'YOUR EMAIL FROM (eg: no-reply <no-reply@dify.ai>)'
-      SMTP_HOST: ''
+      SMTP_SERVER: ''
       SMTP_PORT: 587
       SMTP_USERNAME: ''
       SMTP_PASSWORD: ''