include gdb-gdbserver in docker images

[fche]

RHEL base docker images include the /usr/bin/gdbserver (gdb-gdbserver subrpm), because this enables somewhat straightforward remote debugging of running containers. (gdbserver acts as a remote debugging agent.) Please add this tiny piece into the centos base images too.

[serhei]

Some reasons why gdbserver might be desirable in the CentOS images:

1. Consistency with the enterprise RHEL images.

2. I want to debug a process in the container, but my debuginfo is outside the container and quite big, so I don't want to bother copying it into the container. I launch gdbserver within the container and connect to it with gdb from my laptop, where the debuginfo is available.

3. I want to use a different debugging tool that speaks gdbserver protocol. There is work in-progress or upcoming to make other tools (e.g. strace) be able to connect to a gdbserver on a separate host. Then I would be able to strace a process in the container without actually having to install strace into the container.