Add note about DOCKER_HOST environment variable

Author: rgaiacs

docs/source/start.md

@@ -24,6 +24,8 @@ Follow [Docker's official installation steps](https://docs.docker.com/get-starte
 
 Follow [Podman's official installation steps](https://podman.io/docs/installation).
 
+And configure the `DOCKER_HOST` environment variable following [Podman's official procedure](https://podman-desktop.io/docs/migrating-from-docker/using-the-docker_host-environment-variable#procedure).
+
 (install)=
 
 ## Install `repo2docker`