added comment on variable-substitution not happening in docker-compose env_file
added comment on variable-substitution not happening in docker-compose env_file

byran/cyber-dojo-web,byran/cyber-dojo-web,cyber-dojo/web,byran/cyber-dojo-web,byran/cyber-dojo-web,byran/cyber-dojo-web,byran/cyber-dojo-web,byran/cyber-dojo-web,cyber-dojo/web,cyber-dojo/web,byran/cyber-dojo-web,byran/cyber-dojo-web,cyber-dojo/web