Fix variable name

TestTestNet/mining-node/install.sh

@@ -51,9 +51,13 @@ prepare_homedir() {
 
 add_user_to_docker_group() {
     # based on https://askubuntu.com/questions/477551/how-can-i-use-docker-without-sudo
+    echo "=====> add_user_to_docker_group"
     sudo groupadd docker
-    sudo gpasswd -a "${ADMIN_USER}" docker
+    sudo gpasswd -a "${ADMIN_USERNAME}" docker
     newgrp docker
+    echo "===== Groups: "
+    groups
+    echo "<===== add_user_to_docker_group"
 }
 
 install_ntpd() {