strace a container

gistfile1.txt

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
2cdef4202c1c        cg-ruby-sem_sem     "rails c"           19 minutes ago      Up 19 minutes       3000/tcp            cg-ruby-sem_sem_run_73d6529b9b23

$ cat Dockerfile.debug
FROM alpine
RUN apk update && apk add strace
CMD ["strace", "-p", "1"]

$ docker build -f Dockerfile.debug -t strace .

$ docker run -it --pid=container:2cdef4202c1c --net=container:2cdef4202c1c --cap-add sys_admin --cap-add sys_ptrace strace
strace: Process 1 attached
read(7,