Add additional Docker image build with debug info included.

[Ferroin]

Summary

• Adds a new build argument for our Docker images, DEBUG_BUILD, which when set changes the default CFLAGS for the build to include debug info and optimize for debugging (currently, it changes the default from -O2 -pipe to -Og -ggdb -pipe).
• Split the CFLAGS handling in the Docker image to it’s own script, simplifying the above-mentioned change.
• Add a new build job to the Docker GHA workflow to build and publish an image with this argument set, using the image name netdata/netdata-debug. This will match tagging and versioning with netdata/netdata.

Test Plan

The new build argument can be tested by checkout out the PR branch and running a docker build with --build-arg DEBUG_BUILD=1. The resultant image should have full debugging info in the Netdata Agent binary.

The split CFLAGS handling can be tested by manually running packaging/docker/gen-cflags.sh. If CFLAGS is set in the environment, it should print the contents of that variable, otherwise if DEBUG_BUILD is set to a non-empty string, it should print -Og -ggdb -pipe, otherwise it should print -O2 -pipe.

Additional Information

Fixes: #13341