1. Get the hot pid for something running inside the container
container: sleep 876
host: ps aux|grep sleep|grep 876

2. Create netns directory
host: sudo mkdir -p /var/run/netns/

3. Create the name space softlink:
host: ln -sfT /proc/$pid/ns/net /var/run/netns/$container_id

4. Run ip netns command to access this name space. For example:
host: ip netns exec $container_id ip a