forced cron installation

Dockerfile

 FROM robinthoni/postgres-multiarch:9.6
 
+RUN apt-get update && apt-get -y install\
+        cron &&\
+        apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+
 COPY ./backup/pg_backup.config /etc/pg_backup.config
 
 COPY ./backup/pg_backup_rotated.sh /usr/local/bin/pg_backup_rotated.sh