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>

docker-compose.yml

@@ -1,6 +1,7 @@
 services:
   nextcloud:
-    image: nextcloud:latest
+    build: .
+    image: nextcloud-custom:latest
     restart: unless-stopped
     ports:
       - "127.0.0.1:8888:80"
@@ -56,7 +57,7 @@ services:
       - nextcloud-net
 
   cron:
-    image: nextcloud:latest
+    image: nextcloud-custom:latest
     restart: always
     volumes_from:
       - nextcloud