Ship darnit itself in a hermetic container to eliminate host-environment as an audit-nondeterminism source

[mlieberman85]

Summary

Short-term shippable companion to the determinism set (#418-#421):
publish darnit itself as a pinned container image so audit output is
not influenced by the operator's host environment (Python version,
gh CLI version, git version, tree-sitter version, LLM SDK version,
etc.).

This does not help the audited repo's reproducibility. It closes a
different loop: darnit's own audit runs are more reproducible across
operators when everyone runs the same pinned binaries.

Scope

• Add a Dockerfile at packaging/Dockerfile (or wherever feature 012
  already lands packaging assets):
  • Pin Python interpreter version (matching workspace target).
  • uv sync --frozen for lockfile-pinned deps.
  • Install pinned versions of gh CLI, git, syft, cosign as
    needed by darnit's exec handlers.
  • Non-root user; minimal working-dir layout.
  • ENTRYPOINT ["darnit"].
• Publish under the same release surfaces feature 012 already uses
  (GHCR image, cosign-signed, SBOM via syft).
• Document usage in the release runbook (mount repo as
  /workspace, pass GH_TOKEN, ANTHROPIC_API_KEY env vars).
• Add a smoke-test in CI that pulls the image and runs a mock audit
  against a fixture repo to catch regressions.

Non-goals for v0

• Automatic invocation from darnit audit (the container is a
  distribution shape, not a runtime dependency of the CLI).
• Devcontainer image for darnit development (separate concern;
  the shipped image is for audit execution).
• Rootless / distroless base variants (nice-to-have; revisit if
  operators ask).
• Reproducible-build guarantees on the image itself (bit-for-bit
  container reproducibility is RE-02.01 HermeticBuild: evolve beyond grep-based heuristic toward verifiable hermeticity #227 / feature 012's territory).

Constitution alignment

• II. Conservative-by-Default: eliminating host-env drift from
  audit output preserves the "same commit -> same verdict" invariant
  that the determinism issues (Determinism Tier 1: normally deterministic operations must be deterministic across runs #418 in particular) are chasing at
  the code-path level.

Related

• 012-packaging-distribution -- reuses the existing release
  infrastructure (SBOM, cosign, GHCR).
• RE-02.01 HermeticBuild: evolve beyond grep-based heuristic toward verifiable hermeticity #227 (HermeticBuild -- evolve beyond grep-based heuristic) --
  the image itself is a small proof point that darnit knows how to
  ship a hermetic build.
• Determinism Tier 1: normally deterministic operations must be deterministic across runs #418 (Determinism Tier 1) -- companion; a hermetic runner
  closes the host-side lever the code-side determinism fixes leave
  open.
• Design conversation 2026-09-05 -- filed as short-term
  companion to the environment-generation sieve issue (filed
  alongside this one) while that ambitious sieve is still in
  investigation.