wip: Implement a scheduler to run recurring tasks (such as z-push-gabsync) (#123)

* add scheduler container for gabsync
Adds a general scheduler container to trigger tasks within containers.
Also adds gabsync to zpush image
* make scheduler dynamic
execute each cron job once at startup to see if they would succeed
* remove services scripts from core container (now is part of the scheduler)
* add "CRONDELAY" for tasks that should not be executed at startup
* add documentation
* clear out crontab at startup

scheduler/Dockerfile

@@ -0,0 +1,17 @@
+FROM docker:18.09.1
+
+RUN apk --no-cache add bash
+
+ENV SUPERCRONIC_VERSION 0.1.8
+RUN wget https://github.com/aptible/supercronic/releases/download/v${SUPERCRONIC_VERSION}/supercronic-linux-amd64 \
+    -O /usr/local/bin/supercronic \
+    && chmod +x /usr/local/bin/supercronic
+
+ENV DOCKERIZE_VERSION v0.6.1
+RUN wget https://github.com/jwilder/dockerize/releases/download/$DOCKERIZE_VERSION/dockerize-alpine-linux-amd64-$DOCKERIZE_VERSION.tar.gz \
+    && tar -C /usr/local/bin -xzvf dockerize-alpine-linux-amd64-$DOCKERIZE_VERSION.tar.gz \
+    && rm dockerize-alpine-linux-amd64-$DOCKERIZE_VERSION.tar.gz
+
+COPY start.sh /usr/local/bin/
+
+CMD ["start.sh"]

scheduler/start.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+
+set -eo pipefail
+
+cronfile=/etc/crontab
+
+# purge existing entries from crontab
+true > "$cronfile"
+
+for cronvar in ${!CRON_*}; do
+	cronvalue=${!cronvar}
+	echo "Adding $cronvalue to crontab"
+	echo "$cronvalue" >> "$cronfile"
+done
+
+for cronvar in ${!CRONDELAYED_*}; do
+	cronvalue=${!cronvar}
+	echo "Adding $cronvalue to crontab"
+	echo "$cronvalue" >> "$cronfile"
+done
+
+# wait for kopano_server statup to run one-off commands
+dockerize \
+	-wait tcp://kopano_server:236 \
+	-timeout 360s
+echo "creating public store"
+docker exec kopano_server kopano-storeadm -h default: -P || true
+
+# run sheduled cron jobs once
+for cronvar in ${!CRON_*}; do
+	cronvalue=${!cronvar}
+	croncommand=$(echo "$cronvalue" | cut -d ' ' -f 6-)
+	echo "Running: $croncommand"
+	$croncommand
+done
+
+exec supercronic /etc/crontab