Add Docker Cron documentaion

docs/markdown/docker.md

@@ -13,6 +13,38 @@ docker run --rm \\
   autorestic backup -va -c /data/.autorestic.yaml
 ```
 
+## Cron
+
+To use cron with the docker image,
+you have 2 environment variables.
+`AUTORESTIC_INITIAL_ARGS` and `CRON_CONFIG_DIR`
+
+- `AUTORESTIC_INITIAL_ARGS` is arguments used for the initial autorestic command on container start up, if you don't set it, autorestic would show available commands.
+
+For example:
+```
+AUTORESTIC_INITIAL_ARGS=backup -va -c /.autorestic.yaml
+```
+Would mean `autorestic backup -va -c /.autorestic.yaml` on container startup.
+
+- `CRON_CONFIG_DIR` to enable Cron, you need to set this to the in-contaier directory of the config file you want to use with Cron.
+
+### Example
+
+```
+version: '3.3'
+
+services:
+  autorestic:
+    image: cupcakearmy/autorestic:latest
+    environment:
+      - AUTORESTIC_INITIAL_ARGS=backup -va -c /.autorestic.yaml
+      - CRON_CONFIG_DIR=/.autorestic.yaml
+    volumes:
+      - ./autorestic.yaml:/.autorestic.yaml
+```
+This would run `autorestic backup -va -c /.autorestic.yaml` on container startup, and check for any backups due in /.autorestic.yaml.
+
 ## Rclone
 
 For rclone you will have to also mount the rclone config file to `/root/.config/rclone/rclone.conf`.