feat: define missing user in docker container

Define the missing ${USER} variable in the entrypoint-user.sh script to ensure it is set correctly in the Docker container.

This commit adds a new line of code that sets the ${USER} variable to "${USERNAME}".

The purpose of this change is to properly configure the game server by checking if the specified file exists and setting up necessary variables accordingly.

entrypoint-user.sh

@@ -12,6 +12,9 @@ exit_handler_user() {
 echo -e "Loading exit handler"
 trap exit_handler_user SIGQUIT SIGINT SIGTERM
 
+# Define ${USER} as missing in docker container
+USER="${USERNAME}"
+
 # Setup game server
 if [ ! -f "${GAMESERVER}" ]; then
   echo -e ""