feat: publish Docker image to GHCR on each release

[mostronatorcoder]

Description

Automate Docker image builds and publish them to GitHub Container Registry (GHCR) on each tagged release.

Current state

The project has a Dockerfile that builds a production image using node:20-alpine, but images are only built locally. Users must clone the repo and build manually.

Proposal

Add a GitHub Actions workflow that:

1. Triggers on tag push (v*)
2. Builds the Docker image using the existing Dockerfile
3. Pushes to ghcr.io/lnp2pbot/bot:vX.Y.Z and ghcr.io/lnp2pbot/bot:latest
4. Uses multi-platform builds (amd64 + arm64) for broader deployment support

Benefits

• Operators can pull pre-built images instead of building from source
• Consistent, reproducible deployments
• Version-pinned images for production stability
• Enables easier Docker Compose / Kubernetes deployments