Release 0.21.1

# History

## 0.21.1

Install/update via the bootstrap script:

```shell
wget https://raw.githubusercontent.com/dokku/dokku/v0.21.1/bootstrap.sh
sudo DOKKU_TAG=v0.21.1 bash bootstrap.sh
```

### Documentation

- #4060: @josegonzalez Add warning about the 0.21.0 release

debian/control

@@ -1,5 +1,5 @@
 Package: dokku
-Version: 0.21.0
+Version: 0.21.1
 Section: web
 Priority: optional
 Architecture: amd64