docs: add docs

docs/run-as-non-root-user.md

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+# Run as non-root user
+
+By default the container runs the backup script as root user. There are few things you need to set to run the container as non-root user if you wish to do so.
+
+You can use the non-root user and group `backuptool` created by the image, uid and gid are `1100`.
+
+<br>
+
+
+
+## Backup
+
+1. Make sure that the rclone config file in the mounted `vaultwarden-rclone-data` volume is writable by the user.
+
+```shell
+# enter the container
+docker run --rm -it \
+  --mount type=volume,source=vaultwarden-rclone-data,target=/config/ \
+  --entrypoint=bash \
+  ttionya/vaultwarden-backup:latest
+
+# modify the rclone config file owner in the container
+chown -R 1100:1100 /config/
+
+# exit the container
+exit
+```
+
+2. Start the container with proper parameters.
+
+**With Docker Compose**
+
+```shell
+# docker-compose.yml
+services:
+  backup:
+    image: ttionya/vaultwarden-backup:latest
+    user: 'backuptool:backuptool'
+    ...
+```
+
+**With Automatic Backups**
+
+```shell
+docker run -d \
+  ...
+  --user backuptool:backuptool \
+  ...
+  ttionya/vaultwarden-backup:latest
+```
+
+<br>
+
+
+
+## Restore
+
+Do the restore normally, nothing special.