base_url <> get_image(name, struct, version). base_url is defined several lines above that :
base_url = System.get_env("BACKEND_URL"). Now, if you look at your env/local.env file, the environment variable "BACKEND_URL" is not defined. So even if you source it, it does not respond with a valid variable value and so you get the message that there is an argument error. Just define the variable, source the file, and everything should work fine (it did with me).