結論
$ docker inspect -f '{{.State.Pid}}' <ssを実行したいコンテナの名前かID>
12345 <- そのコンテナのPIDが表示される
$ docker run -it --rm --privileged --pid=host debian nsenter -t <上記で取得したPID> -m -u -n -i bash
root@2a403fe42474:/# ss -tpn | cat
State Recv-Q Send-Q Local Address:Port Peer Address:Port
ESTAB 0 0 172.19.0.6:57376 172.19.0.5:5432 users:(("gunicorn",328,12))
ESTAB 0 0 172.19.0.6:45504 172.19.0.3:5672 users:(("gunicorn",216,16))
ESTAB 0 0 172.19.0.6:57968 172.19.0.4:27017 users:(("st2notifier",48,7))
ESTAB 0 0 172.19.0.6:45528 172.19.0.3:5672 users:(("st2actionrunner",92,9))
ESTAB 0 0 172.19.0.6:50750 172.19.0.2:6379 users:(("st2notifier",48,10))
ESTAB 0 0 172.19.0.6:57974 172.19.0.4:27017 users:(("st2actionrunner",92,6))
ESTAB 0 0 172.19.0.6:57970 172.19.0.4:27017 users:(("gunicorn",214,13))
ESTAB 0 0 172.19.0.6:57976 172.19.0.4:27017 users:(("st2actionrunner",100,6))
ESTAB 0 0 172.19.0.6:58070 172.19.0.4:27017 users:(("gunicorn",216,21))
ESTAB 0 0 172.19.0.6:57952 172.19.0.4:27017 users:(("gunicorn",216,13))
ESTAB 0 0 172.19.0.6:45508 172.19.0.3:5672 users:(("st2resultstrack",45,9))
ESTAB 0 0 172.19.0.6:57366 172.19.0.5:5432 users:(("mistral-server",294,4))
ESTAB 0 0 172.19.0.6:57978 172.19.0.4:27017 users:(("st2actionrunner",92,7))
ESTAB 0 0 172.19.0.6:45544 172.19.0.3:5672 users:(("gunicorn",216,18))
ESTAB 0 0 172.19.0.6:58080 172.19.0.4:27017 users:(("python",348,8))
ESTAB 0 0 172.19.0.6:45526 172.19.0.3:5672 users:(("st2actionrunner",92,10))
ESTAB 0 0 172.19.0.6:57966 172.19.0.4:27017 users:(("st2notifier",48,6))
ESTAB 0 0 172.19.0.6:45532 172.19.0.3:5672 users:(("st2actionrunner",100,10))
ESTAB 0 0 172.19.0.6:57986 172.19.0.4:27017 users:(("st2rulesengine",51,6))
ESTAB 0 0 172.19.0.6:57954 172.19.0.4:27017 users:(("gunicorn",202,13))
ESTAB 0 0 172.19.0.6:45496 172.19.0.3:5672 users:(("mistral-server",294,7))
ESTAB 0 0 172.19.0.6:57960 172.19.0.4:27017 users:(("st2garbagecolle",40,6))
ESTAB 0 0 172.19.0.6:57964 172.19.0.4:27017 users:(("st2garbagecolle",40,7))
ESTAB 0 0 172.19.0.6:45548 172.19.0.3:5672 users:(("gunicorn",216,23))
ESTAB 0 0 172.19.0.6:57944 172.19.0.4:27017 users:(("st2sensorcontai",39,6))
ESTAB 0 0 172.19.0.6:57956 172.19.0.4:27017 users:(("gunicorn",202,14))
ESTAB 0 0 172.19.0.6:57950 172.19.0.4:27017 users:(("st2resultstrack",45,7))
ESTAB 0 0 172.19.0.6:57972 172.19.0.4:27017 users:(("gunicorn",214,14))
ESTAB 0 0 172.19.0.6:57988 172.19.0.4:27017 users:(("st2rulesengine",51,7))
ESTAB 0 0 172.19.0.6:45540 172.19.0.3:5672 users:(("st2sensorcontai",39,10))
ESTAB 0 0 172.19.0.6:57948 172.19.0.4:27017 users:(("st2resultstrack",45,6))
ESTAB 0 0 172.19.0.6:45530 172.19.0.3:5672 users:(("st2actionrunner",100,9))
ESTAB 0 0 172.19.0.6:45514 172.19.0.3:5672 users:(("st2notifier",48,9))
ESTAB 0 0 172.19.0.6:45492 172.19.0.3:5672 users:(("mistral-server",294,5))
ESTAB 0 0 172.19.0.6:57946 172.19.0.4:27017 users:(("st2sensorcontai",39,7))
ESTAB 0 0 172.19.0.6:50780 172.19.0.2:6379 users:(("gunicorn",216,22))
ESTAB 0 0 172.19.0.6:45564 172.19.0.3:5672 users:(("st2actionrunner",92,11))
ESTAB 0 0 172.19.0.6:45494 172.19.0.3:5672 users:(("mistral-server",294,6))
ESTAB 0 0 172.19.0.6:45536 172.19.0.3:5672 users:(("st2rulesengine",51,9))
ESTAB 0 0 172.19.0.6:57980 172.19.0.4:27017 users:(("st2actionrunner",100,7))
ESTAB 0 0 172.19.0.6:45490 172.19.0.3:5672 users:(("st2sensorcontai",39,9))
ESTAB 0 0 172.19.0.6:45594 172.19.0.3:5672 users:(("st2notifier",48,12))
ESTAB 0 0 172.19.0.6:45554 172.19.0.3:5672 users:(("python",348,14))
ESTAB 0 0 172.19.0.6:45538 172.19.0.3:5672 users:(("st2rulesengine",51,10))
ESTAB 0 0 172.19.0.6:57362 172.19.0.5:5432 users:(("gunicorn",327,11))
ESTAB 0 0 172.19.0.6:58078 172.19.0.4:27017 users:(("python",348,7))
ESTAB 0 0 172.19.0.6:57958 172.19.0.4:27017 users:(("gunicorn",216,14))
プロセスのユーザーがちゃんと表示される。
# ワンライナー
docker run -it --rm --privileged --pid=host debian nsenter -t $(docker inspect -f '{{.State.Pid}}' <ssを実行したいコンテナの名前かID>) -m -u -n -i bash
Docker for Macでも問題なく使える。