Nsenter Invalid Argument

Problem: you tried to enter docker namespace but got an error:

nsenter: reassociate to namespace 'ns/net' failed: Invalid argument

This can be caused by docker daemon started with MountFlags=slave

Solution: Use a double nsenter:

nsenter -m -t $(cat /var/run/docker.pid) \
    nsenter --net=/var/run/docker/netns/${network_id} ip link