Add opt-in Docker registry existence checks before rewriting tags #13

Summary

Offer an optional verification step that confirms proposed Docker image tags exist on Docker Hub, GHCR, or ECR before writing changes, reducing the risk of pushing broken images.

Why

Some registries lag behind upstream Python releases. Automatically switching to a tag that is not yet published breaks deployments. An existence check allows teams to gate upgrades until registries catch up.

What needs to happen

• Add an opt-in input (e.g. verify_registry: true|false) with support for Docker Hub, GHCR, and ECR.
• Implement registry-specific probes using the minimal API calls required (respecting rate limits and auth).
• Integrate with the run summary / PR body to note when a tag was skipped for missing registry support.
• Provide configuration hooks for private registries or credentials.
• Add tests using mocked registry responses and fixtures.
• Document the new flag and behavior in README.

References

• Roadmap capability item: "Registry existence check".

Acceptance criteria

• When enabled, the action skips rewrites if a Docker tag is absent and reports which registry failed.
• Default behavior remains unchanged when the flag is off.
• Tests exercise success, failure, and mixed-registry scenarios.