FIX ruby tools.

Dockerfile

@@ -8,7 +8,7 @@ ENV REDMINE_SEND_REMINDERS_DAYS=0 \
 	REDMINE_SEND_REMINDERS_CRON_EXPR="0 7 * * 1-5"
 
 RUN apt-get update \
-	&& apt-get install -y cron libffi-dev libffi-dev \
+	&& apt-get install -y cron ruby-dev libffi-dev \
 	# Clean-up \
 	&& apt-get clean \
 	&& rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* ~/.cache \