Add Cron container for backing up the slapd data.

crond/.dockerignore

+*
+!crontab

crond/Dockerfile

+FROM docker:19.03 as docker
+
+# hadolint ignore=DL3006
+FROM adarnimrod/cron as supersonic
+COPY --from=docker /usr/local/bin/docker /usr/local/bin/
+COPY --chown=root:root crontab /crontab
+# hadolint ignore=DL3002
+USER root

crond/README.md

+# crond
+
+> Cron container image.
+
+## Description
+
+This container periodically runs the slapd backup script. The script runs in the
+slapd container so the `dockerd` socket is bind mounted to this container and
+the commands are executed via `docker exec` in the other containers. Thus,
+outside of the context of this setup this container doesn't make much sense.
+
+## License
+
+This software is licensed under the MIT license (see `LICENSE.txt`).
+
+## Author Information
+
+Nimrod Adar, [contact me](mailto:nimrod@shore.co.il) or visit my [website](
+https://www.shore.co.il/). Patches are welcome via [`git send-email`](
+http://git-scm.com/book/en/v2/Git-Commands-Email). The repository is located
+at: <https://www.shore.co.il/git/>.

crond/crontab

+@daily docker exec ldap_ldap_1 backup

docker-compose.yml

@@ -2,6 +2,12 @@
 ---
 version: '3.5'
 services:
+  crond:
+    build:
+      context: crond/
+    volumes:
+      - /run/docker.sock:/run/docker.sock
+
   ldap:
     build:
       cache_from: