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>

.dockerignore

@@ -0,0 +1,17 @@
+# Données Nextcloud
+data/
+db/
+backups/
+logs/
+
+# Docker
+docker-compose*.yml
+.env*
+
+# Git
+.git/
+.gitignore
+
+# Documentation
+README.md
+*.md