Optimize Docker build stages and runtime layering

[DenWin]

Goal

Improve Docker layer reuse and keep build-only tooling out of the production image without changing ConvertX conversion behavior.

Implemented locally

• Split development and production Bun dependencies into independent stages.
• Copy package manifests before application source to preserve dependency caching.
• Download Bun in a disposable stage.
• Download VTracer in a disposable stage.
• Keep curl and unzip out of the final runtime image.
• Use Docker TARGETARCH for explicit amd64 and arm64 asset selection.
• Harden downloads with curl failure handling.
• Correct the eslint.config.ts build-context ignore entry.
• Invoke the compiled application directly with Bun.
• Pass docker build --check with no warnings.
• Pass git diff --check.

Remaining validation

• Complete a production image build.
• Run representative converter smoke tests.
• Compare image size and docker history with the previous Dockerfile.
• Validate both linux/amd64 and linux/arm64.

Notes

The current working-tree implementation intentionally retains debian:testing-slim and the complete runtime package set until compatibility is proven separately.