Add advanced server configuration, custom launch parameters, and GHCR builds

[KrisCris]

Summary

This PR adds more flexibility for advanced and modded Core Keeper server deployments while preserving the existing default behavior.

• Add opt-in OVERRIDE_SERVER_CONFIG support that generates ServerConfig.generated.json at startup.
• Expose MAX_NUMBER_PACKETS_SENT_PER_FRAME and NETWORK_SEND_RATE for network tuning.
• Add CUSTOM_LAUNCH_PARAMS for server arguments not covered by existing environment variables.
• Preserve quoted values and support standalone flags and flag/value pairs in custom parameters.
• Add a GitHub Actions workflow that builds and publishes multi-platform Docker images to GHCR.

Motivation

Some dedicated-server settings—particularly network snapshot tuning and mod-specific launch flags—cannot be configured through the existing environment variables.

These changes make those settings available without requiring users to modify the container scripts or maintain a separate image. All new behavior is opt-in, so existing deployments continue to work unchanged.