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.

[gayathrii12]

1. The PR says NOTES.md documents the before/after image-size comparison, but that file is not included in the changed files. Please add it, or update the PR description to remove that claim.

2)Include the actual size comparison and explain the benefit

[PuneethM-06]

• Imagine size has been updated in the ticket with screenshot
• updated PR summary