When following this walkthrough, a $CONFIG_DIR environment variable is used without explicitly setting it. Is there a way to associate this with the project using gcloud config set rather than create a local environment variable of that name?
Nope. You'll need to create a local environment variable. gcloud config set is only used for gcloud specific settings, not environment settings. $CONFIG_DIR isn't used by gcloud itself, it's used by your shell to expand the files being passed into gcloud.