build(docker): reliable apt mirror and correct build context

- The default deb.debian.org Cloudflare CDN throttled to ~150 kB/s from
  this network and repeatedly dropped the gcc-12/cpp-12 fetch, failing
  the image build twice. Switch apt to mirror.yandex.ru and add
  Acquire retry/timeout options so the system-dependency layer
  (OCRmyPDF, Tesseract rus+eng, Ghostscript, qpdf) builds reliably.
- Copy pyproject.toml, README.md, alembic.ini and the app/scripts
  packages before "pip install -e ." so hatchling's editable build can
  validate the readme and packages (previously failed with
  "Readme file does not exist: README.md").
- Drop README.md from .dockerignore so the build context includes it.
- Ignore local preview launch config (frontend/.claude, .claude/launch.json).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -13,4 +13,3 @@ tests
 .ruff_cache
 .idea
 .vscode
-README.md