For sites using the terminal command to run updates only do file and composer operations no Drupal booting in during that script

Problem/Motivation

Right now if a site uses the auto-script directly to allow write protecting the side from the web server user, the server account that runs auto-script will have to boot drupal and write to the file system. It would better if this privileged user didn't have cause Drupal to bootstrap.

Steps to reproduce

Proposed resolution

One way to do this would be invoke most of the update process from cron but then have script that is set to run only does

1. Stages the files
2. Runs the composer commands on the stage
3. Copies file back over to the active directory
4. Deletes stage directories after they have been used or abandoned

Everything else should be done by another user.

Solution

If the site is set up to use the auto-update script executed by the server cron job, have this script only do the tasks above.
Everything else should be done by script that executed directly from cron

Remaining tasks

User interface changes

API changes

Data model changes