This commit includes the following improvements:
- Document that the `create-image.sh` script (and by extention the
`update-preview-server.sh` script) need to have access to a `yarn`
executable.
- Add a note on cron jobs running in non-interactive, non-login shells
(which affects their execution context and have different behavior vs
running the same commands in an interactive, login shell).
- Change the Node.js and `yarn` installation instructions to ensure the
`yarn` executable will be available on the `PATH` and not require an
interactive, login shell (as happens, for example, when installing it
via [nvm](https://github.com/nvm-sh/nvm)). This makes it easier to set
up a cron job that runs the `update-preview-server.sh` script.
NOTE: The equivalent updates have been made on the GCE VM that hosts the
PR preview server.
PR Close#37015
2d065a32ec