feat(docker): add multi-stage CLI image and CI smoke test

[nitishagar]

Closes #11.

Summary

Adds a minimal, multi-stage Docker image so docker run --rm -v "$PWD":/work anydoc convert <file> produces the same Markdown as the local cargo run --example convert. No bindings, no service, no registry push.

• Builder rust:1.88-bookworm compiles examples/convert.rs with --release --locked (PDF support is included by default — pdf-inspector is a plain dependency, no feature flags exist in Cargo.toml).
• Runtime debian:bookworm-slim carries only the convert binary + ca-certificates; WORKDIR /work, ENTRYPOINT ["convert"]. Documents are read from a mounted volume — nothing is baked in.
• .dockerignore keeps target/, node_modules/, binding build artifacts, and agent scratch dirs out of the context.
• CI adds a docker job that builds the image and runs one smoke conversion against tests/fixtures/docx/text.docx, so the Dockerfile cannot silently rot.

Why debian over alpine

pdf-inspector and its deps (lopdf, rayon, ttf-parser) are pure Rust — there are no native deps that block musl — but glibc matches the CI runner (ubuntu-2404) and a fully-static musl validation is deferred to a follow-up rather than shipped unvalidated for v1.

Verification

• cargo fmt --all --check ✅
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✅
• cargo build --release --locked --example convert ✅
• docker build -t anydoc . ✅ on darwin/arm64
• Image size: 33 MB (target was < 200 MB)
• CLI parity: docker run … convert tests/fixtures/docx/text.docx output is byte-identical to local convert (1316 bytes both).
• Negative paths: no-args → prints usage: + exit 1; missing file → clear error + exit 1.
• Branch rebased on current main.

Scope / follow-ups (out of scope here)

• No stdin (-) support: examples/convert.rs always reads a file path — it has no stdin branch. This is deferred to feat(cli): add a Rust anydoc binary for cargo install #29 (the standalone src/bin/anydoc.rs CLI), which owns real stdin. Documented as a spec gap, not silently dropped.
• arm64 in CI: CI builds amd64 only (QEMU emulation is slow); the Dockerfile is arch-neutral, verified locally on arm64.
• Registry publishing (GHCR/Docker Hub): maintainer decision, not included.
• Non-root USER hardening: deferred to a follow-up (the plan calls this out).

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