Switch to Debian Buster, install/uninstall gpg to import repo keys, improve cleanup

Dockerfile

@@ -14,7 +14,7 @@
 # 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
+FROM debian:buster-slim
 
 # Prevent the packages installation to halt.
 ENV DEBIAN_FRONTEND noninteractive
@@ -29,9 +29,13 @@ 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" \
+    apt-get update && \
+    apt-get install -y gnupg && \
+    echo "deb http://deb.seadrive.org buster main" \
         > /etc/apt/sources.list.d/seafile.list && \
     bash /import-seafile-apt-key.sh && \
+    apt-get remove -y gnupg && \
+    apt-get autoremove -y && \
     apt-get update && \
     apt-get install \
         -o Dpkg::Options::="--force-confold" \
@@ -42,10 +46,11 @@ RUN mkdir -p /etc/apt/sources.list.d/ && \
     apt-get autoclean \
         -o APT::Clean-Installed=true && \
     rm \
-        -f \
+        -rf \
             /var/log/fsck/*.log \
             /var/log/apt/*.log \
             /var/cache/debconf/*.dat-old \
+            /var/lib/apt/lists/* \
             /import-seafile-apt-key.sh && \
     mkdir /volume/ && \
     echo "seafuser" > /etc/cron.allow && \