Enable ipv4 forwarding for docker managed bridge

[bencord0]

When enabling packet forwarding for a bridge managed by docker, set the sysctl for the bridge.

The previous behaviour would set the forwarding flag for the global default. This patch unifies the behaviour between ipv4 and ipv6, since the ipv4 code path to set the forwarding behaviour explicitly for the bridge being managed.

---

Context: I can't find any design documents or tests around this area for what the expected behaviour is supposed to be. However, for me I see the following behaviour.

Using docker 17.04.0 with flannel 0.6.2 in host-gw mode ( {"Network": "10.0.0.0/16", "Backend": {"Type": "host-gw"}})

Docker is started (via systemd) using the following resulting command line.
/usr/bin/dockerd -H fd:// --bip=10.0.80.1/24 --ip-masq=true --mtu=1500

Start a container and grab it's IPAddress
docker inspect $(docker run -d nginx)|grep IPAddress

From another host (that is part of the flannel network, and has it's routes setup correctly)
ping $IPADDRESS or curl $IPADDRESS.

Expected results:
I can successfully ping/curl the container from another host.

Actual results:
The IP packets are for forwarded from the host interface, enp1s0, through to the docker0 interface. Using tcpdump, I can follow the packets entering the host's network interface (enp1s0 -> docker0), but response packets are not forwarded back (docker0 -> enp1s0).

This can be verified with
cat /proc/sys/net/ipv4/conf/docker0/forwarding -> 0

---

With this patch packet forwarding can be enabled after taking into account https://github.com/docker/libnetwork/blob/4dd9c0603ab3c0cf34e165f9678511c18b4f4f17/drivers/bridge/bridge.go#L756

/usr/bin/dockerd -H fd:// --bip=10.0.80.1/24 --ip-masq=true --mtu=1500 --icc=false

Now, forwarding is enabled.
cat /proc/sys/net/ipv4/conf/docker0/forwarding -> 1
and I can ping/curl the container from another host.

[bencord0]

You can also replicate the test without needing a second host using the -I (capital i) flag to ping.
e.g. ping -I enp1s0 $IPADDRESS

[bencord0]

flannel-io/flannel#872 May be of interest as an alternative workaround.