ci: add Docker build job with SHA-based image tagging

[puneeth-grinds]

PR Summary

• Added a build job to the GitHub Actions CI pipeline.
• Configured the job to run only after both lint and test complete successfully.
• Built the Docker image inside CI using the full Git commit SHA as the image tag.
• Added conditional latest tagging for builds running on the main branch.
• Listed local Docker images after the build to verify the generated tags in the workflow logs.

Why

• Ensures Docker images are successfully built as part of CI before any publish step.
• Uses immutable SHA-based tags for traceability and reproducible builds.
• Prevents the latest tag from being created on feature branches.
• Provides build visibility by displaying generated image tags in the workflow logs.