README.md

@@ -11,6 +11,10 @@ This is the primary repository for the ESA OPS-SAT experiment.
 
 ## Build
 
+You might need to set the [`CROSS_CONTAINER_ENGINE`](https://github.com/cross-rs/cross/wiki/FAQ#explicitly-choose-the-container-engine)
+and [`CROSS_ROOTLESS_CONTAINER_ENGINE`](https://github.com/cross-rs/cross/blob/main/docs/environment_variables.md#configuring-cross-with-environment-variables)
+variables manually before calling cross.
+
 ```sh
 cross build
 ```