fix: use docker-compose.override.yml for server-specific config

- Remove :ro and network: host from base docker-compose.yml
- Add docker-compose.override.yml.example with network/healthcheck template
- Add docker-compose.override.yml to .gitignore

This allows git pull without conflicts on the server. Server-specific
settings (external networks, healthchecks) go in the override file.

docker-compose.yml

@@ -2,7 +2,6 @@ services:
   lightbox:
     build:
       context: .
-      network: host
     container_name: immich-folio
     restart: unless-stopped
     ports: