dist/docker: make docker-entrypoint.py pass signals to supervisord
dist/docker: make docker-entrypoint.py pass signals to supervisord

Stopping docker currectly didn't pass the signals to supervisord,
hence scylla wasn't gracefully shutdown.

Fixes #6150

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla