Add multi-stage Dockerfile for container deployment

Three-stage build:
- assets: Node 22 + Vite + Tailwind CSS 4 → static/dist/main.css
- python-deps: uv sync --frozen --no-dev against pyproject + uv.lock,
  with the issue-core path dependency satisfied via a BuildKit
  named context (--build-context issue-core=...)
- runtime: python:3.12-slim-bookworm + libpq5 + curl, non-root 'app'
  user, collectstatic at build time, gunicorn on :8000,
  /health/ HEALTHCHECK every 30s

Adds gunicorn>=22 to project dependencies (was missing).

Build:
  docker build --build-context issue-core=/home/worsch/issue-core \
    -t gitea.coulomb.social/coulomb/vergabe-teilnahme:<tag> .

Smoke-verified: container reports (healthy) and /health/ returns
{"status": "ok"} without a database connection.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,33 @@
+.git
+.gitignore
+.dockerignore
+Dockerfile
+
+__pycache__
+*.py[cod]
+.venv
+.pytest_cache
+.ruff_cache
+.mypy_cache
+.coverage
+htmlcov
+
+node_modules
+static/dist
+staticfiles
+media
+
+*.log
+*.sqlite3
+.issue-facade
+
+.env
+.env.*
+docker-compose*.yml
+
+.claude
+.custodian-brief.md
+workplans
+wiki
+LICENSE
+README.md