Use it to troubleshoot already running containers :
TARGET_CONTAINER_ID=<target_container_name_or_id>
docker run -it \
--pid=container:$TARGET_CONTAINER_ID --net=container:$TARGET_CONTAINER_ID \
--cap-add sys_admin --privileged \
pryz/troubleshoot sh
Then :
strace -p 1
lsof -p 1
htop
...