The solution
It was hard to find an answer but after trying all possibilities, I found a simple one :
- Open : Oracle VM VirtualBox Manager
- Select the VM used by docker (by default it's "default")
- Right click on it -> close -> shutdown
- Right click on it (again) -> settings -> Network -> card 1 -> advanced -> ports forwarding
- Now you click in the right top corner (+) to add a forwarding rule
- Give it the name you want (for me it was "ftp")
- Choose a protocol (for me it was TCP cause FTP use TCP)
- Let the 2 IP fields empty (to let default)
- Set the 2 ports (for me it was 21, cause is the standard ftp port)
- Press OK
- OK again
- That's it, you just have to (re)start docker and for the port you chose, you can use the IP address of your computer (which is running (the ftp server in) docker)
I think the solution can help some people, it's why I post it here.
(and a big thanks to Davey Chu who help me to fix this problem)