Reduce image CVEs by switching to minimal base and cleanup build chain

[talkraghu]

Summary
This PR improves the build reproducibility and mitigates potential risk from upstream CVEs by switching the base build image from gcc:latest to gcc:13.

Background
The previous Dockerfile used gcc:latest as the base image for building DRBD utilities. Using a floating tag can inadvertently introduce new vulnerabilities over time and lead to unpredictable build behavior. Pinning the image to gcc:13 ensures a stable, auditable, and more secure build environment.

Testing Performed
Rebuilt the image locally with gcc:13.
Scanned using grype: 0 Critical CVEs and 9 High CVEs, all originating from libxml2 and python3 in the UBI 9 runtime base image.

Functional verification:
drbd-shutdown-guard --help displays usage correctly.
drbd-shutdown-guard's initcontainer at satellite-node pod is completing well.

[talkraghu]

With this change, the scan on the docker image shows NO critical CVE's

There are a few HIGH CVE's though (listed below) (which should be fine)
python3 3.9.21-2.el9 rpm CVE-2025-4517 High 27.56 < 0.1
python3-libs 3.9.21-2.el9 rpm CVE-2025-4517 High 27.56 < 0.1
python3 3.9.21-2.el9 rpm CVE-2025-4138 High 26.27 < 0.1
python3-libs 3.9.21-2.el9 rpm CVE-2025-4138 High 26.27 < 0.1
libxml2 2.9.13-9.el9_6 rpm CVE-2025-49794 High 21.33 < 0.1
python3 3.9.21-2.el9 rpm CVE-2024-12718 High 19.88 < 0.1
python3-libs 3.9.21-2.el9 rpm CVE-2024-12718 High 19.88 < 0.1
libxml2 2.9.13-9.el9_6 rpm CVE-2025-49796 High 16.24 < 0.1
libxml2 2.9.13-9.el9_6 rpm CVE-2025-49795 High 16.24 < 0.1