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>

.dockerignore

@@ -0,0 +1,11 @@
+.git
+__pycache__
+*.pyc
+*.pyo
+config.json
+screenshots/
+.claude/
+CLAUDE.md
+*.swp
+*.swo
+.DS_Store