Install cron by bitnami manual

Dockerfile

 FROM bitnami/phpbb:3.3.8
 MAINTAINER Andrej Ramašeuski <andrej.ramaseuski@pirati.cz>
 
-RUN set -ex; \
-        mkdir -p /var/lib/apt/lists/partial /var/lib/apt/lists/auxfiles /var/lib/apt/mirrors/partial  /var/lib/apt/mirrors/periodic && \
-        apt-get update && \
-        DEBIAN_FRONTEND=noninteractive \
-        apt-get install --no-install-recommends -y \
-        \
-        cron
-
+USER 0
+RUN install_packages cron
 COPY crontab /etc/cron.d/
+USER 1001
+
 COPY phpbb /opt/bitnami/phpbb
 CMD [  ( cron -f -l 8 & ) & "/opt/bitnami/scripts/apache/run.sh" ]