fix: use entrypoint.sh in Dockerfile for startup validation

[gordonmurray]

Summary

• docker/entrypoint.sh validates that DATA_PATH exists and is readable before starting uvicorn, but the Dockerfile bypassed it with a direct CMD
• When DATA_PATH was missing or unreadable, users got a Python traceback instead of a clear error message
• The entrypoint script already uses exec so uvicorn replaces the shell process and receives signals (SIGTERM etc.) correctly on container shutdown

Changes

One file changed: docker/Dockerfile

• COPY docker/entrypoint.sh into the image
• chmod +x the script
• Replace CMD ["python", "-m", "uvicorn", ...] with ENTRYPOINT ["/app/entrypoint.sh"]

Test

# Missing DATA_PATH
$ docker run --rm -e DATA_PATH=/nonexistent lance-data-viewer:test
ERROR: Data path /nonexistent does not exist or is not mounted

# Unreadable DATA_PATH
$ docker run --rm -v /tmp/unreadable:/data:ro lance-data-viewer:test
ERROR: Data path /data is not readable

# Happy path
$ docker run --rm -p 8080:8080 -v ./data:/data:ro lance-data-viewer:test
Starting Lance Viewer on port 8080...
Data path: /data
INFO:     Started server process

Fixes #21