cannot ipv6 address in ptf container

```
net.ipv6.conf.eth0.disable_ipv6 = 1
root@ca8b83f02540:~# sysctl -w net.ipv6.conf.eth0.disable_ipv6=0     
sysctl: setting key "net.ipv6.conf.eth0.disable_ipv6": Read-only file system
root@ca8b83f02540:~# ip -6 addr add fc00::2/126 dev eth0           
RTNETLINK answers: Permission denied
```