feat: convert to multi-stage build to reduce image size (closes #14)

[PuneethM-06]

Summary

Implemented a multi-stage Docker build for linux-sysmonitor to separate application preparation from the runtime environment.

Changes

• Added a dedicated builder stage to prepare the application.
• Added a separate runtime stage containing only the application and required runtime dependencies.
• Used COPY --from=builder to copy the prepared application into the final image.
• Preserved existing container behavior while demonstrating the multi-stage build pattern.

Verification

• Built the optimized image successfully.
• Verified the container executes successfully and produces the same output as the previous version.
• Compared image sizes before and after optimization.
• Documented the image size comparison in NOTES.md.