Fixes all issues raised in PR #47 review:
1. Add .dockerignore to prevent:
- Real .env secrets baked into image layers
- Host node_modules (glibc) overwriting alpine (musl) → ABI mismatch
- .git, reports/, evidence/ bloating the image
2. Fix compose host-binding:
- Remove T3MP3ST_HOST=127.0.0.1 override (was breaking port forwarding)
- Server now binds to 0.0.0.0 in-container (DOCKER=true from Dockerfile)
- Host mapping stays 127.0.0.1:3333:3333 for security
3. Add critical security warning in DOCKER.md:
- Explain exposure risk of 0.0.0.0 binding + disabled Host-header guard
- Document safe vs unsafe deployment patterns
- Warn against network_mode: host and -p 3333:3333 without 127.0.0.1
4. Update base image:
- node:18-alpine → node:22-alpine (Node 18 EOL)
- Pin by digest @sha256:16e22a550... (prevent floating tag drift)
5. Fix healthcheck in compose:
- Was always returning success (process.exit(0))
- Now actually tests /api/health endpoint
- Add start_period: 40s to align with Dockerfile
6. Fix port typos in DOCKER.md:
- 3000 → 3333 in troubleshooting examples
- Update all example snippets to node:22-alpine with digest
