notes: add commands to backup and restore volumes

notes/docker.md

@@ -0,0 +1,21 @@
+# Backup and restore volumes
+
+Backup a volume
+
+```sh
+docker run \
+  --rm \
+  --volumes-from source_container_name \
+  -v $(pwd):/backup \
+  busybox tar cvf /backup/backup.tar /path/to/volume
+```
+
+Restore volume from a backup
+
+```sh
+docker run \
+  --rm \
+  --volumes-from target_container_name \
+  -v $(pwd):/backup \
+  busybox sh -c "cd /path/to/volume && tar xvf /backup/backup.tar --strip 1"
+```