avoid write dump result to stdout

[trim21]

I'm tring to dump mysql to s3 and I find it write entire content of sql (output of mysqldump) to stdout, make it hard to find out what's happening

[trim21]

I find the DumpName is not actually used in the mysql/Dump, but it's expected to write output to it, is this a bug?

https://github.com/appleboy/docker-backup-database/blob/dd44d09e32f42e69d533441c0a829950c53ad2a6/pkg/dbdump/mysql/mysql.go#L18