Add Docker support for zero-dependency setup

[Joxx0r]

Summary

Closes #17

• Dockerfile: Multi-stage build — Go stage compiles Zoekt binaries, Node.js slim stage runs the service. All prerequisites bundled.
• docker-compose.yml: One-command startup with volume mounts for project data and persistent database.
• config.docker.json: Template config using /projects mount paths and 0.0.0.0 bind address.
• Cross-platform fixes: killExistingService() and _killStaleWebservers() now handle Linux (lsof/pkill) alongside Windows (netstat/taskkill).
• Docker detection: setup.js detects Docker via DOCKER=1 env or /.dockerenv and skips WSL/Go prerequisite checks.
• CI workflow: Builds Docker image on PRs touching Docker files, pushes to GHCR on version tags.

Usage

# 1. Copy and edit the config template
cp config.docker.json config.json
# Edit config.json to match your volume mount paths

# 2. Add volume mounts in docker-compose.yml
#    - D:/p4/games/MyProject:/projects/MyProject:ro

# 3. Start
docker compose up

Test plan

• docker build . completes successfully (multi-stage with Go + Node)
• docker compose up starts the service and Zoekt webserver
• Health check endpoint responds at http://localhost:3847/health
• Zoekt binaries are found natively (no WSL fallback attempted)
• npm run setup inside container shows "Docker: YES (all prerequisites bundled)"
• Volume-mounted project files are indexed correctly
• Existing Windows/WSL workflow is unaffected by the cross-platform changes

🤖 Generated with Claude Code

[github-actions]

@claude Please review this PR. Focus on:

• Code quality and potential bugs
• Security issues
• Test coverage
• Documentation completeness

Use the opus model for thorough analysis.