Docker build check along with PR Quality Checker

[torrid-fish]

Goal

Fail PRs that break the Docker image build before they reach main, so the CD pipeline (.github/workflows/cd.yml) never ships a broken release to GHCR.

[torrid-fish]

Resolved by PR #63, which adds .github/workflows/docker-build.yml. On any PR touching Dockerfile, docker-compose.yml, pyproject.toml, uv.lock, scripts/download_unidic.sh, or the workflow itself, it builds the image (no push) and runs an offline smoke test (--network none) confirming the baked dictionaries resolve. A broken Dockerfile, a stale uv.lock, or a failed dict bake fails the PR before merge. Closes when #63 merges.