That seems like a mistake in the design of the compose file mentioned (not using the “{{ domain }}” pattern in the LEMMY_UI_LEMMY_EXTERNAL_HOST environment variable) in the docs and a case of the documentation not being explicit about what needs to be changed (they refer to that variable in the README of the Lemmy UI repo when you read about the config file mentioned in the docs).
That seems like a mistake in the design of the compose file mentioned (not using the “{{ domain }}” pattern in the LEMMY_UI_LEMMY_EXTERNAL_HOST environment variable) in the docs and a case of the documentation not being explicit about what needs to be changed (they refer to that variable in the README of the Lemmy UI repo when you read about the config file mentioned in the docs).