Publish a hardened Docker image for Harper

[kriszyp]

Produce a hardened Docker image variant for Harper (in addition to the standard image), with a minimal base (distroless or equivalent) — eliminating shell, package manager, and other tooling commonly used in container exploits.

Background

Discussed in the internal Slack thread. Tradeoff is that a hardened image removes features Harper currently uses at runtime (or operators use for debugging), so it requires careful evaluation and may need code changes to keep working without them.

Asks

• Audit which Linux userspace tools Harper currently invokes at runtime (subprocess calls, package downloads, etc.) and decide which can be removed vs. replaced.
• Produce a hardened image (e.g. harperfast/harper:X.Y.Z-hardened) in the standard release pipeline alongside the existing image.
• Document the tradeoffs: what's removed, what operators can/can't do inside the container, debug workflow with no shell.

Acceptance criteria

• Hardened image is published as a release artifact.
• All integration tests pass against the hardened image.
• Documented in the docker/install guide.

Notes

• GPU image (CUDA) is a separate concern (see CORE-3077) — likely can't be hardened in the same way.
• The existing Dockerfile is in Dockerfile at the repo root.

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Tracked in Jira: CORE-3023

🤖 Filed by Claude on behalf of Kris.