use of Docker addon
You get access to docker. There is an env var injected into the app. See https://docs.cloudron.io/packaging/addons/#docker .
Generally, don't use it. If you are using it, it means you are trying to access docker which is a bit of security hole. This was created mostly out of necessity and maybe for "playing" . But once you give an app access to docker, it can pretty much do anything on host.