Periodic garbage collection.

- Add a crond image.
- Add a weekly Cron job to run garbage collection.

crond/.dockerignore

+*
+!crontab

crond/Dockerfile

+# hadolint ignore=DL3006
+FROM registry.shore.co.il/cron as supersonic
+# hadolint ignore=DL3002
+USER root
+# hadolint ignore=DL3018
+RUN apk add --update --no-cache docker-cli

crond/README.md

+# crond
+
+> Cron container image.
+
+## Description
+
+Periodically run the registry's garbage collection. It should be improved to
+first put the registry in a read-only mode or disable the Nginx proxy or
+something.
+
+## 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://git.shore.co.il/explore/>.

crond/crontab

+@weekly docker exec registry_registry_1 registry garbage-collect /etc/docker/registry/config.yml

docker-compose.yml

@@ -16,6 +16,7 @@ services:
     restart: always
     volumes:
       - "registry:/var/lib/registry"
+
   reg:
     command:
       - "server"
@@ -34,6 +35,13 @@ services:
     image: r.j3ss.co/reg:v0.16.1
     restart: always
 
+  crond:
+    build:
+      context: crond/
+    restart: always
+    volumes:
+      - /run/docker.sock:/run/docker.sock
+
 volumes:
   registry: