Running phan the super simple docker way
Your filesystem layout needs to look like (the core folder can have any name):
core/ core/skins/ core/skins/Timeless
Once you're in the Timeless directory, run:
composer update docker run -v $(pwd)/../..:/mediawiki --env THING_SUBNAME=skins/Timeless -it --rm docker-registry.wikimedia.org/releng/mediawiki-phan:latest
Depending on permissions, you might need to run the docker command as root (with sudo). After you make some changes, just run the docker command again.