fix(docker): default untagged images to latest

[efegokdemir]

Summary

Default bare Docker image references to the latest tag before sending them to the Engine API.

Related Issue

Fixes #4029

Changes

• Add :latest when the final image path segment has neither a tag nor a digest.
• Preserve explicit tags and digests.
• Treat registry ports such as localhost:5000/image as registry syntax, not as an image tag.
• Add focused unit coverage for all reference forms.

Testing

• git diff --check
• cargo fmt --all -- --check — not run: Cargo is not installed in the local environment.
• cargo test -p openshell-driver-docker docker_image_reference --lib — not run: Cargo is not installed in the local environment.

Checklist

• Change is limited to Docker image reference normalization and tests
• Commit is signed off
• Focused Rust test run requires CI or a Rust toolchain

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