Raspberry: Add notes about adding user to "docker" group

Belongs to https://github.com/orgs/solectrus/discussions/2769#discussioncomment-8311046

guide/external-server/README.md

@@ -228,9 +228,20 @@ If Docker is not installed, install it:
 ```console
 curl -fsSL https://get.docker.com -o get-docker.sh
 sudo sh get-docker.sh
-sudo usermod -aG docker Pi
 ```
 
+Don't forget to add your user to the `docker` group, so you don't need to use `sudo` for every Docker command.
+
+```console
+sudo groupadd docker
+sudo usermod -aG docker $USER
+```
+
+Log out and log back in so that your group membership is re-evaluated.
+
+Details:
+https://docs.docker.com/engine/install/linux-postinstall/
+
 We don't need Docker Compose on the Raspi, because just one container will be started.
 
 ### b) Install and run SENEC Collector