feat(docker): modernize Dockerfile, add runit init script, and provide docker-compose setup

- Add python3-libvirt to apt and configure --system-site-packages for zero-compilation build
- Introduce 10_webvirtcloud_init.sh to handle container startup, migrations, and SQLite persistence
- Exclude host settings.py in .dockerignore for clean builds from settings.py.template
- Add docker-compose.yml with persistent data and SSH volume mounts
- Document Docker Compose deployment in README.md

.dockerignore

@@ -21,6 +21,7 @@
 **/*.log
 **/console/cert.pem*
 **/dhcpd.*
+webvirtcloud/settings.py
 
 # IDEs and OS
 **/.idea