The docker-compose.yml contained in core refers to DEVELOPERS.md which is not included in the standard package:

# Please see DEVELOPERS.md for help
#
# Contributions to this file are welcome but please note that this file is
# minimal by design, with the idea to make it easily extensible via
# docker-compose.override.yml. For help with doing that, please see
# DEVELOPERS.md

This is potentially confusing. I am happy to try and improve the situation, but would appreciate guidance on approach; I see three:

1. adapt docker-compose.yml to something like "... see DEVELOPERS.md (if present) or https://gerrit.wikimedia.org/g/mediawiki/core/+/HEAD/DEVELOPERS.md"
2. include DEVELOPERS.md in all mediawiki packagings
3. remove DEVELOPERS.md from core and host information exclusively on the wiki, link there