We're installing our Pantheon D8 sites in /web to **provoke** merge conflicts and thus prevent us from appliying Pantheon's one click updates from the dashboard. We're using composer to manage our D8 sites, which is incompatible with the one-click updates. (So this isn't a bug, it's a feature!) Happily, we're pretty sure that Pantheon have moved all their magical secret sauce into modules and the settings file. In theory, none of it is in core anymore.
Instead:
1. Pull a copy of the site onto your local
2. Navigate to the /web directory of your site
3. Use `composer update drupal/core --with-dependencies` to update using composer
- Unfortunately, Composer sometimes installs nested .git directories, which make Pantheon very unhappy. So after you've updated, you need to find and remove any nested .git directories:
4. Run `find . -name .git -type d -prune` to get a list of any new .git directories hiding in the bowels of your install. You might see something like: `/web/vendor/afdafaf/adsfasdfdas/.git`
5. Remove them eg. `rm -r /web/vendor/afdafaf/adsfasdfdas/.git`
6. Then add and commit all changes and push them back up to Pantheon
7. Make sure to update the database and clear caches on Pantheon!