docker run --user

$ docker run                        --rm -it busybox /bin/sh -c id
uid=0(root) gid=0(root) groups=10(wheel)

$ docker run --user 1234            --rm -it busybox /bin/sh -c id
uid=1234 gid=0(root)

$ docker run --user 1234:5678       --rm -it busybox /bin/sh -c id
uid=1234 gid=5678

$ docker run --user rene:superusers --rm -it busybox /bin/sh -c id
docker: Error response from daemon: unable to find user rene: no matching entries in passwd file.