CI: Link GitHub releases with versioning

[Shmayro]

Motivation

• Improve the GitHub Actions pipeline to use newer action versions and produce deterministic Docker image tags and OCI labels for releases and main branch builds.
• Prevent pushing images for pull requests and centralize image name configuration via an IMAGE_NAME env var.
• Document release-based versioning and add a release badge to the README so users understand image tagging semantics.

Description

• Renamed workflow to "Build and Publish Docker Image" and changed release trigger type to published while adding IMAGE_NAME in the workflow env.
• Upgraded actions to actions/checkout@v4, docker/setup-buildx-action@v3, docker/login-action@v3, and docker/build-push-action@v5.
• Added a Prepare Docker metadata step that runs an embedded Python script to compute tags and labels (exposed via steps.meta.outputs) based on event type, ref, and SHA, including edge, sha-<short>, semver-derived tags, and latest for stable releases.
• Made Docker login conditional to skip on pull requests and configured build-push-action to only push when github.event_name != 'pull_request', and to use the generated tags and labels outputs.
• Updated README.md to add a GitHub release badge, a Table of Contents entry, and a Versioning and Releases section that explains how vX.Y.Z releases map to Docker tags.

Testing

• No automated tests were executed as part of this change.

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Codex Task