feat: add Dockerfile for self-hosting

[elio-Wang]

• Add multi-stage Dockerfile (node-server preset)
• Add .dockerignore
• Allow overriding git revision and Nitro preset via env vars
• Document Docker usage in README

Closes #19

🔗 Linked issue

resolves #19

🔄 Context

Currently the server is hardcoded to the netlify_edge preset with no official way to self-host. Users who want to deploy on their own infrastructure need a Docker image.

📚 Description

Add a multi-stage Dockerfile that builds the server with the node-server Nitro preset, producing a minimal production image containing only Node.js and the compiled .output directory.

Changes:

• Dockerfile: 3-stage build (deps → build → runtime), final image based on node:22-slim
• .dockerignore: exclude node_modules, .git, .output, etc.
• server/nitro.config.ts: allow NITRO_PRESET and GIT_REVISION env var overrides, with try-catch fallback for git rev-parse in environments without .git
• README.md: add Docker build & run instructions with environment variable reference