Add Docker Compose and Podman support for containerized deployment

Adds Dockerfile, docker-compose.yml, and .dockerignore to enable
running ADS-Bit in a container with host networking for LAN receiver
auto-scanning. Bind mounts persist config, sprites, and backgrounds
across container recreation. Updates README with usage instructions.

Closes #4

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,15 @@
+services:
+  ads-bit:
+    build: .
+    container_name: ads-bit
+    network_mode: host
+    restart: unless-stopped
+    environment:
+      - PYTHONUNBUFFERED=1
+    volumes:
+      # Persist config (copy config.json.example to config.json before first run)
+      - ./config.json:/app/config.json
+      # Persist custom sprite uploads
+      - ./images:/app/images
+      # Persist custom theme backgrounds
+      - ./backgrounds:/app/backgrounds