This readme has information on patterns for developing.
Ensure you're using dev not prod:
firebase use <DEV_PROJECT_ID>
Make sure you have admin credentials (https://firebase.google.com/docs/functions/local-shell):
Generate a key and downlaod it to e.g. ~/.firebase-keys/KEY.json
#the path has to be fully specified
export GOOGLE_APPLICATION_CREDENTIALS="/Users/<USERNAME>/.firebase-keys/KEY.json"
Download your server-side config:
firebase functions:config:get > .runtimeconfig.json
Run the shell:
firebase functions:shell
Within the shell, call the cloudfunction:
screenshot.get('CARD-ID')
Note that there screenshot.js's DISABLE_SCREENSHOT_CACHE can be set to true during development. Also note that puppeteer can be launched with headless:false and slowMo:100 to view what's going on.