diff --git a/Dockerfile b/Dockerfile
index 8cf17129b4948b98ddb1736d80ac59b8ed44d964..b7cbbbe6378901e3e9bb341dc4ebe4205eea49d1 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -16,4 +16,5 @@ USER nobody
 CMD [ "supersonic", "/crontab" ]
 HEALTHCHECK CMD pgrep supersonic
 RUN supersonic -test /crontab
+ONBUILD COPY crontab /crontab
 ONBUILD RUN supersonic -test /crontab
diff --git a/README.md b/README.md
index d0d9f87a1dda668e4cfb15c1501ed47ab529f884..4e5030be8560395e93f2a8cfe79c1c9f04a27457 100644
--- a/README.md
+++ b/README.md
@@ -18,13 +18,13 @@ FROM alpine:latest
 COPY --from=supersonic /usr/local/bin/supersonic /usr/local/bin/
 ```
 
-The other pattern is building on top of this image to run some periodic tasks
-like so:
+The other pattern is building on top of this image to run some periodic tasks.
+The `ONBUILD` commands will copy the crontab file and validate it. Just copy
+whatever scripts and install whatever packages you need, like so:
 
 ```
 FROM registry.shore.co.il/cron
 RUN apk add --update --no-cache aws-cli
-COPY crontab /crontab
 COPY script /usr/local/bin/
 ```