This would be because you’re using a different version of Discourse in development and docker production.
This commit moved the helpers from discourse/lib/helpers to discourse-common/lib/helpers:
https://github.com/discourse/discourse/commit/be1d74d20772e3a4ed2b3daed9700249024a133f
So if you update your development environment, and change the first line of your code to:
import { registerHelper } from 'discourse-common/lib/helpers';
then everything should work.