Go with the cron alternative for the staging and stable releases

Dockerfile

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+# Docker Seafile client, help you mount a Seafile library as a volume.
+# Copyright (C) 2019, flow.gunso@gmail.com
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+FROM debian:jessie-slim
+
+# Prevent the packages installation to halt.
+ENV DEBIAN_FRONTEND noninteractive
+# Create the seafile client user.
+ENV UNAME=seafuser
+ENV UID=1000
+ENV GID=1000
+
+# Copy over the Docker related files.
+COPY utils/build/import-seafile-apt-key.sh /
+COPY assets/cron/docker-entrypoint.sh /entrypoint.sh
+
+# Safely import Seafile APT key, then install both seafile-cli and supervisord.
+RUN mkdir -p /etc/apt/sources.list.d/ ;\
+    echo "deb http://deb.seadrive.org jessie main" \
+        > /etc/apt/sources.list.d/seafile.list ;\
+    bash /import-seafile-apt-key.sh ;\
+    apt-get update ;\
+    apt-get install \
+        -o Dpkg::Options::="--force-confold" \
+        -y \
+            seafile-cli \
+            cron ;\
+    apt-get clean ;\
+    apt-get autoclean \
+        -o APT::Clean-Installed=true ;\
+    rm \
+        -f \
+            /var/log/fsck/*.log \
+            /var/log/apt/*.log \
+            /var/cache/debconf/*.dat-old \
+            /import-seafile-apt-key.sh ;\
+    mkdir /volume/ ;\
+    echo "seafuser" > /etc/cron.allow ;\
+    echo "*/20 * * * * /bin/bash /home/seafuser/seafile-healthcheck.sh" \
+        > /var/spool/cron/crontabs/seafuser ;\
+    groupadd -g $GID -o $UNAME ;\
+    useradd -m -u $UID -g $GID -o -s /bin/bash $UNAME
+
+# Copy over the required files for Seafile/SupervisorD.
+COPY assets/cron/seafile-healthcheck.sh /home/seafuser/seafile-healthcheck.sh
+COPY assets/cron/seafile-entrypoint.sh /home/seafuser/entrypoint.sh
+
+ENTRYPOINT ["/bin/bash", "/entrypoint.sh"]