I had a look at the code! Seems like the launcher uses local_discourse/app (wich not exists) as image if the variable $run_image is empty! This has the following code:
cat $config_file | $docker_path run $user_args --rm -i -a stdin -a stdout $image ruby -e \
"require 'yaml'; puts YAML.load(STDIN.readlines.join)['run_image']"