notes: add information about clean up docker/overlay2 directory

notes/docker.md

@@ -19,3 +19,23 @@ docker run \
   -v $(pwd):/backup \
   busybox sh -c "cd /path/to/volume && tar xvf /backup/backup.tar --strip 1"
 ```
+
+# Clean overlay2 folder
+
+```sh
+docker image ls --all --format "{{.ID}}" | xargs -n1 docker inspect | egrep '"Id"|/merged'
+
+```
+
+You'll see something like the following:
+
+```
+"Id": "sha256:f8fa7183309d043e93c78224f93e8bce028be5ac331b994e3f3654b8980e7d61",
+    "MergedDir": "/var/lib/docker/overlay2/8eb1a12796c337c8a60f00d6721c26a0dd7ceba66f28f3a66be0cb5b08505561/merged",
+```
+
+And now it safe to remove merged dir
+
+```sh
+rm -rf /var/lib/docker/overlay2/8eb1a12796c337c8a60f00d6721c26a0dd7ceba66f28f3a66be0cb5b08505561
+```