enter for debug

I have run an nginx container...

docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
6d67de07731d        nginx               "nginx -g 'daemon ..."   40 minutes ago      Up 40 minutes       80/tcp, 443/tcp     epic_goldberg

I want to use strace for debug:
docker run -it --pid=container:6d67de07731d --net=container:6d67de07731d --cap-add sys_admin --cap-add sys_ptrace bash

I can see the nginx process:

bash-4.4# ps auxww | grep nginx
root         1  0.0  0.2  31868  5116 ?        Ss   16:54   0:00 nginx: master process nginx -g daemon off;
104          6  0.0  0.1  32256  2836 ?        S    16:54   0:00 nginx: worker process
root        58  0.0  0.0  11128   928 ?        S+   17:10   0:00 grep nginx

I can change into the filesystem of the nginx process:

bash-4.4# cd /proc/1/root/.
bash-4.4# find . -name nginx
./etc/default/nginx
./etc/init.d/nginx
./etc/logrotate.d/nginx
./etc/nginx
./var/cache/nginx
./var/log/nginx
./usr/lib/x86_64-linux-gnu/perl5/5.20/auto/nginx
./usr/lib/nginx
./usr/sbin/nginx
./usr/share/lintian/overrides/nginx
./usr/share/doc/nginx
./usr/share/nginx

# You can also strace the nginx process

bash-4.4# apk update
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/community/x86_64/APKINDEX.tar.gz
v3.5.1-29-ga981b1f149 [http://dl-cdn.alpinelinux.org/alpine/v3.5/main]
v3.5.1-29-ga981b1f149 [http://dl-cdn.alpinelinux.org/alpine/v3.5/community]
OK: 7957 distinct packages available
bash-4.4# apk add strace
(1/1) Installing strace (4.14-r0)
Executing busybox-1.25.1-r0.trigger
OK: 12 MiB in 16 packages
bash-4.4# strace -p 1
strace: Process 1 attached
rt_sigsuspend([], 8^Cstrace: Process 1 detached