Feat/docker security

[PuneethM-06]

Summary

This PR hardens the linux-sysmonitor Docker image by applying Docker security best practices and documenting the results of a Trivy vulnerability assessment.

Changes Made

• Performed a baseline Trivy vulnerability scan on the Docker image.
• Pinned the Debian base image to a specific version (debian:12.11-slim) for reproducible builds.
• Reduced the runtime attack surface by installing packages with --no-install-recommends.
• Removed apt package metadata after installation to keep the image lean.
• Added a dedicated non-root user (appuser) and configured the container to run without root privileges.
• Rebuilt the hardened Docker image and performed a follow-up Trivy scan.
• Added SECURITY.md documenting the baseline scan, hardening steps, final scan, and comparison of results.

Security Notes

The container now follows several Docker security best practices, including running as a non-root user and minimizing installed packages. The remaining vulnerabilities originate from Debian base packages and runtime dependencies. Several are reported by Trivy as will_not_fix, affected, or fix_deferred, indicating that no upstream patched version was available at the time of scanning.

Closes #16.