docker pull andrewvos/docker-proftpd
docker run -p 21:21 -p 20:20 -e USERNAME=username -e PASSWORD=password -v `pwd`/ftp:/ftp andrewvos/docker-proftpd

ftp -p localhost 21
Name (0.0.0.0:andrew): username
Password:
ftp> ls

Note: if you use boot2docker on Mac OSX, be sure to connect to the VM when testing like,

ftp -p $(boot2docker ip) 21.