【docker】curl実行後死亡するコンテナ

cat Dockerfile                              15:26:32
FROM alpine:3.7
RUN apk update
RUN apk add curl
CMD curl -s -X POST -H 'Content-Type: application/json' -d '{"text":"hello!"}' https://hooks.slack
.com/services/TMGL7M997/B01NLKW8AFJ/xxxxxxxxxxxxxxxxx -o /dev/null