Replace Singularity with Docker; add production deployment setup

- Remove Singularity.def; add Dockerfile (python:3.11-slim + ImageMagick,
  non-root user, installs gallery package with dev extras)
- Add .dockerignore to keep image lean
- Rewrite .gitlab-ci.yml: build→test→publish stages using Docker-in-Docker;
  push per-commit SHA tag and promote to :latest on main
- Add docker-compose.yml: gallery-generator + nginx services sharing a
  named volume; configurable GENERATE_INTERVAL env var
- Add deploy/nginx.conf: gzip, security headers, correct caching policy
  (immutable for assets, no-store for HTML)
- Add deploy/entrypoint.sh: runs gallery generate on startup then loops
  on GENERATE_INTERVAL; exits cleanly when interval is 0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,20 @@
+FROM python:3.11-slim
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends imagemagick \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY pyproject.toml .
+COPY gallery/ gallery/
+COPY tests/ tests/
+COPY deploy/entrypoint.sh deploy/entrypoint.sh
+
+RUN pip install --no-cache-dir ".[dev]"
+
+RUN useradd -r -s /bin/false gallery
+USER gallery
+
+ENTRYPOINT ["gallery"]
+CMD ["generate"]