Docker: a script to turn WP into subfolder install

We sometimes need to test in WordPress sub-folder install.

Would be useful to have a script for the Jetpack-Docker setup, which would turn the setup into a sub-folder install and back to regular root folder install.

https://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

#### Our Docker setup:
https://github.com/Automattic/jetpack/tree/master/docker#readme

#### Helper scripts live here:
https://github.com/Automattic/jetpack/blob/5a27262047656478aaa6fce706e7aa88c6eded5f/package.json#L26-L39

Start with the Docker README and the helper-script entries in package.json at lines 26–39, then use the linked WordPress subdirectory guidance to define both transitions. Done means the Jetpack-Docker setup can switch to a subfolder install and return to the regular root-folder install.

Written by the indexing model from the issue text.