I did same thing as you mentioned. (uncommented the line as you mentioned)
$ cat /etc/sysctl.conf | grep 'net.ipv6.conf.all.forwarding'
Then I did this to apply above change
$ sudo sysctl -p
net.ipv6.conf.all.forwarding = 1
And after that I run
$ docker run -p 8080:8080 rajneesh/testimage
sudo netstat -anp |grep 8080
tcp6 0 0 :::8080 :::* LISTEN 28248/docker.io
but still I am unable to connect with my server from outside the world while there is no filtering mechanism on host machine.
I can see that issue is reported on docker https://github.com/docker/docker/issues/2174
but unable to find any working solution yet.