Add Sysctl container options

[amdfxlucas]

The sysctl parameters of docker containers(services) can be tuned via the sysctls: section in the docker-compose.yml file.
They're already used to enable IPv4 forwarding (containers acting as routers).
But with the new option system we can seamlessly make SysCtl params regular options which can have different values per node.
This allows i.e. to increase UDP Read/Write buffer sizes on nodes ..

my_node_name:
      ...
    sysctls:
       - net.ipv4.ip_forward=1
       - net.ipv4.udp_rmem_min=${MY_NODE_NAME_UDP_RMEM_MIN}
       - net.ipv4.udp_wmem_min=${MY_NODE_NAME_UDP_WMEM_MIN}

This can even be done on a per Interface level:
Set sysctls for a container's interface (--driver-opt)
sysctl settings that start with net.ipv4. and net.ipv6. can be set per-interface using --driver-opt label com.docker.network.endpoint.sysctls. The name of the interface must be replaced by IFNAME.

To set more than one sysctl for an interface, quote the whole value of the driver-opt field, remembering to escape the quotes for the shell if necessary. For example, if the interface to my-net is given name eth3, the following example sets net.ipv4.conf.eth3.log_martians=1 and net.ipv4.conf.eth3.forwarding=0.

docker network connect --driver-opt="com.docker.network.endpoint.sysctls=net.ipv4.conf.IFNAME.log_martians=1,net.ipv4.conf.IFNAME.forwarding=0" multi-host-network container2
Note

Network drivers may restrict the sysctl settings that can be modified and, to protect the operation of the network, new restrictions may be added in the future.

[amdfxlucas]

merged upstream seed-labs#293