Document debug information workflow with debuginfod

[serhei]

Describe how to extract debug symbols from an OpenShift container build process and use them in a debugging session with GDB (or how to access debug symbols for system packages in a container):

• debug symbols are served by debuginfod-server https://sourceware.org/elfutils/Debuginfod.html
• GDB outside the container connects to debuginfod-server
• GDB outside the container connects to gdbserver inside the container

Generalize to other debugging tools if possible.

[serhei]

(GDB debuginfod support is upcoming in version 9.2)

(There may be a more convoluted procedure to identify build-id and use debuginfod-client to fetch corresponding debuginfo, then specify it for GDB.)