Hello,
I have install images ubuntu into docker and install lamp server.
After install proftpd, i have docker forward port 21 but i have docekr forward 5021 --> 21.
My router config is correct. If i docker forward 21:21 the FTP transfert and connection is OK. If docker forward 5021:21 fillezilla display an error : connection is OK but impossible to retrieve the contents of the file.
I think port 20 is not forward, so i have forward 20 --> 20 or 5020 --> 20 but always error message …
Help me … 
Sorry for my english 