feat: add custom Docker image with ffmpeg for video previews

- Create Dockerfile extending nextcloud:latest with ffmpeg
- Add .dockerignore to optimize build context
- Update docker-compose.yml to use custom image build
- Modify update.sh to rebuild custom image on updates
- Document custom image setup in README
- Enable video preview generation for .mov, .mp4 files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,7 @@
+FROM nextcloud:latest
+
+# Installer ffmpeg pour les previews vidéo
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    ffmpeg \
+    && rm -rf /var/lib/apt/lists/*