Fix container stop on disconnect

[delfer]

Fixing https://github.com/delfer/docker-alpine-ftp-server/issues/15

Vsftpd moved to backgroud. Added https://github.com/ZentriaMC/pidproxy to monitor main vsftpd process and https://github.com/krallin/tini supervisor to reap zombies and performing signal forwarding.

Starting process list:

PID   USER     TIME  COMMAND
    1 root      0:00 /sbin/tini -- /bin/start_vsftpd.sh
    7 root      0:00 pidproxy /var/run/vsftpd/vsftpd.pid true
   30 root      0:00 vsftpd -opasv_min_port=21000 -opasv_max_port=21010 -opasv_
   34 root      0:00 ps -a

Tested:

• logs - in STDOUT OK
• docker kill - container stopped OK
• docker restart - container restarted OK
• docker exec -ti alpine-ftp killall tini - container stopped OK