fix(api): docker entrypoint use exec

exec into gunicorn instead of spawning gunicorn.

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2331>

api/docker/server.sh

@@ -6,7 +6,7 @@ funkwhale-manage collectstatic --noinput
 funkwhale-manage migrate
 
 # shellcheck disable=SC2086
-gunicorn config.asgi:application \
+exec gunicorn config.asgi:application \
     --workers "${FUNKWHALE_WEB_WORKERS-1}" \
     --worker-class uvicorn.workers.UvicornWorker \
     --bind 0.0.0.0:5000 \