Enabling ipv6 support on docker container network. This is needed (#5418

)

for ipv6 communication between container and host in multi-asic
platforms. Address is assign is private address space of fd::/80
with prefix len selected as 80 so that last 48 bits can be
container mac address and and you prevent NDP neighbor cache
invalidation issues in the Docker layer.

Ref: https://docs.docker.com/config/daemon/ipv6/
Ref:https://medium.com/@skleeschulte/how-to-enable-ipv6-for-docker-containers-on-ubuntu-18-04-c68394a219a2

Signed-off-by: Abhishek Dosi <abdosi@abdosi-ubuntu-vm0.nwp1qucpfg5ejooejenqshkj3e.cx.internal.cloudapp.net>

Co-authored-by: Abhishek Dosi <abdosi@abdosi-ubuntu-vm0.nwp1qucpfg5ejooejenqshkj3e.cx.internal.cloudapp.net>

files/docker/docker.service.conf

@@ -1,3 +1,3 @@
 [Service]
 ExecStart=
-ExecStart=/usr/bin/dockerd -H unix:// --storage-driver=overlay2 --bip=240.127.1.1/24  --iptables=false
+ExecStart=/usr/bin/dockerd -H unix:// --storage-driver=overlay2 --bip=240.127.1.1/24  --iptables=false --ipv6=true --fixed-cidr-v6=fd00::/80