Attaching GDB to nginx running inside docker container.

in-container.log

root@b13239f0d920:/opt/openresty/nginx/conf# ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  21564  2228 ?        Ss   Feb24   0:00 /usr/bin/lua -lluarocks.loader /usr/local/lib/luarocks/rocks/lapis/1.4.3-1/bin/lapis server production
root        20  0.0  0.0   4440   652 ?        S    Feb24   0:00 sh -c /opt/openresty/nginx/sbin/nginx -p "$(pwd)"/'' -c 'nginx.conf.compiled'
root        22  0.0  0.1  54944  4280 ?        S    Feb24   0:00 nginx: master process /opt/openresty/nginx/sbin/nginx -p /opt/openresty/nginx/conf/ -c nginx.conf.compiled
nobody      24  0.0  0.0  54944  1964 ?        S    Feb24   0:00 nginx: cache manager process
nobody     473 94.6 25.9 1107220 1050740 ?     R    02:22 861:17 nginx: worker process is shutting down
root       478  0.1  0.0  18156  1964 ?        Ss   17:30   0:00 bash
root       498  0.0  0.0  15564  1144 ?        R+   17:31   0:00 ps aux
root@b13239f0d920:/opt/openresty/nginx/conf# gdb /opt/openresty/nginx/sbin/nginx 473
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /opt/openresty/nginx/sbin/nginx...done.
Attaching to program: /opt/openresty/nginx/sbin/nginx, process 473
Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.
/opt/openresty/nginx/conf/473: No such file or directory.
(gdb)

outside-container.log

$ sudo gdb /opt/openresty/nginx/sbin/nginx 32176
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
/opt/openresty/nginx/sbin/nginx: No such file or directory.
Attaching to process 32176
/opt/openresty/nginx/sbin/nginx: No such file or directory.