Install fix

Dockerfile

 FROM bitnami/phpbb:3.3.8
 MAINTAINER Andrej Ramašeuski <andrej.ramaseuski@pirati.cz>
-RUN install_packages cron
+
+RUN set -ex; \
+        apt-get update && \
+        DEBIAN_FRONTEND=noninteractive \
+        apt-get install --no-install-recommends -y \
+        \
+        cron
 
 COPY crontab /etc/cron.d/
 COPY phpbb /opt/bitnami/phpbb