ref: reset user to root for installation (#1469)

I'm looking to replace `gosu` with a `USER` instruction in getsentry/snuba#2696 -- this requires us to reset the user back to root before proceeding with `apt` installation

Author: asottile-sentry

cron/Dockerfile

@@ -1,5 +1,6 @@
 ARG BASE_IMAGE
 FROM ${BASE_IMAGE}
+USER 0
 RUN apt-get update && apt-get install -y --no-install-recommends cron && \
     rm -r /var/lib/apt/lists/*
 COPY entrypoint.sh /entrypoint.sh