Create a new Drupal 8 site on Pantheon with a Composer-Based Workflow

Thia is an old article, borrowed from an email sent in the Pantheon power-users email group.

For the most up to date info, see https://pantheon.io/docs/guides/drupal-8-composer-no-ci/

• make a variable for your new site machine name to allow all of the following instructions to be copy/paste.
  export TERMINUS_SITE=steve-d8-manual-composer
   
• Create a new site with Terminus and put it in git mode
  terminus site:create $TERMINUS_SITE "My Composer-Managed D8 site" "empty"
  terminus connection:set $TERMINUS_SITE.dev git
   
• We're using the "empty" upstream here because we specifically don't want the dashboard prompting us to merge in the git-based core updates. We're using Composer to update core.
   
• Make a local copy of example-drops-8-composer
  composer create-project pantheon-systems/example-drops-8-composer:dev-master $TERMINUS_SITE
   
• Initialize a git repo in the local copy of example-drops-8-composer if you didn't keep the existing git history when running composer create-project
  cd $TERMINUS_SITE && git init
   
• Add the Pantheon site as a remote and fetch
  git remote add origin $(terminus connection:info $TERMINUS_SITE.dev --field=git_url)
   
• Remove any git directories within dependencies so that they are not added as submodules
  composer prepare-for-pantheon
   
• Add, commit, push
  git add .
  git commit -m 'Initial commit'
  git push --set-upstream origin master --force
   
• Put the site back in SFTP mode so that the settings.php file is writable during installation.
  terminus connection:set $TERMINUS_SITE.dev sftp
   
• Install Drupal
  terminus drush $TERMINUS_SITE.dev -- site-install -y
   
• Commit the settings.php change
• terminus env:commit $TERMINUS_SITE.dev --message="Installing Drupal"

• Log in to your site and appreciate the splendor of Drupal core
  terminus drush $TERMINUS_SITE.dev -- user-login
   
• Export your config to yml files
  terminus drush $TERMINUS_SITE.dev -- config-export -y
   
• Commit those too
  terminus env:commit $TERMINUS_SITE.dev --message="Config from standard profile of Drupal"
   
• Pull down your code changes
  git pull
   
• Go forth with your Composer-managed site!

There is also "terminus build:project:create" which is meant for setting up the GitHub -> CircleCI -> Pantheon workflow.