fix dockerfile issues and upgrade base images

[msarfaty]

Hey all, just looking to start using this image at work, and need to fix some issues with the latest version that was published about a year ago as it's failing our image import process for using an EOL version of Alpine

Let me know how I can help push this along! Thank you thank you

Summary

Update Docker build configuration for compatibility and best practices.

• Upgrade Go from 1.17 to 1.21 and Alpine from 3.16 to 3.20
• Fix ENTRYPOINT placement: move from base-release to individual release stages to ensure binary exists before container starts
• Add missing --from=builder flag in release stage COPY command
• Use absolute path for WORKDIR to resolve Docker build warnings
• Standardize stage naming (lowercase as → uppercase AS)