When attaching a network to a container, sysctl params should be allowed

[paaguti]

Description

Steps to reproduce the issue:

1. Create two docker networks with IPv6
2. Create a container with the first network, attach the second network and attach to the container

sudo docker run --rm -dit \
     --network net1 \
     --sysctl net.ipv4.ip_forward=1 \
     --sysctl net.ipv6.conf.all.forwarding=1 \
     --sysctl net.ipv6.conf.default.forwarding=1 \
     --sysctl net.ipv6.conf.eth0.forwarding=1 \
     --name my-vlan-debian \
     debian

CONTAINER_ID=$(sudo docker ps -a -q -f name=debian)

sudo docker network connect \
     net2 $CONTAINER_ID

sudo docker attach $CONTAINER_ID

3. Inside the container, check that you have both networks and check the
   /proc/sys/net/ipv6/*/forwarding files

Describe the results you received:

All /proc/sys/net/ipv6/*/forwarding files are set to 1 except /proc/sys/net/ipv6/eth1/forwarding which is set to 0. This blocks IPv6 forwarding between eth0 and eth1.

Describe the results you expected:

I somehow expected this to happen. What I need is a way to have /proc/sys/net/ipv6/eth1/forwarding set to 1. Sort of what you do when you create the container with the --sysctl parameters.

Additional information you deem important (e.g. issue happens only occasionally):

I need to build IPv6 routers as containers in order to implement a DMZ inside Docker

Output of docker version:

student@uc3m:~/Devel$ sudo docker version
Client:
 Version:      18.03.1-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   9ee9f40
 Built:        Thu Apr 26 07:17:14 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.03.1-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   9ee9f40
  Built:        Thu Apr 26 07:15:24 2018
  OS/Arch:      linux/amd64
  Experimental: false

Output of docker info:

student@uc3m:~/Devel$ sudo docker version
Client:
 Version:      18.03.1-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   9ee9f40
 Built:        Thu Apr 26 07:17:14 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.03.1-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   9ee9f40
  Built:        Thu Apr 26 07:15:24 2018
  OS/Arch:      linux/amd64
  Experimental: false

Additional environment details (AWS, VirtualBox, physical, etc.):

Inside a VirtualBox 6.0 with A Debian:

uname -a

Linux my-VBox 4.9.0-6-amd64 #1 SMP Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux