To enable FTP inside containers, you must first make sure the proper modules are loaded on the host:
modprobe ip_conntrack
modprobe ip_conntrack_ftp
Don’t forget to add them to /etc/modules
Them inside the /etc/vz/vz.conf setting file add/modify the following line:
IPTABLES="ipt_REJECT ipt_tos ipt_limit ipt_multiport iptable_filter iptable_mangle ipt_TCPMSS ipt_tcpmss ipt_ttl ipt_length iptable_nat ipt_state ipt_conntrack ip_conntrack_ftp"
Then restart the vz service.