feat: configurable port via PORT environment variable

[gordonmurray]

Summary

• The listening port was hardcoded to 8080 in the entrypoint script
• Some orchestrators and reverse proxy setups prefer the container to listen on a specific port
• Now reads PORT env var (defaults to 8080, fully backwards compatible)

Changes

Two files changed: docker/entrypoint.sh, docker/Dockerfile

• entrypoint.sh: reads PORT env var with ${PORT:-8080} default, passes it to uvicorn
• Dockerfile: adds ENV PORT=8080 as the documented default, healthcheck uses ${PORT} so it works with custom ports

Test

# Default port (unchanged behavior)
$ docker run --rm -p 8080:8080 -v ./data:/data:ro lance-data-viewer
Starting Lance Viewer on port 8080...

# Custom port
$ docker run --rm -e PORT=9090 -p 9090:9090 -v ./data:/data:ro lance-data-viewer
Starting Lance Viewer on port 9090...

Both return 200 on /healthz.

Fixes #25