docker networking namespace not visible in ip netns list

That’s because docker is not creating the reqired symlink:

# (as root)
pid=$(docker inspect -f '{{.State.Pid}}' ${container_id})
mkdir -p /var/run/netns/
ln -sfT /proc/$pid/ns/net /var/run/netns/$container_id

Then, the container’s netns namespace can be examined with ip netns ${container_id}, e.g.:

# e.g. show stats about eth0 inside the container 
ip netns exec "${container_id}" ip -s link show eth0