Docker: Allow WP-CLI to be run as root

[tbradsha]

This PR sets the WP_CLI_ALLOW_ROOT env var so running wp in our Docker environment doesn't require --allow-root every time. It also removes that flag from Docker-specific scripts.

There are some other scripts that Docker likely runs but that might run in other contexts (e.g. CI), so I've left those alone.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

1. jetpack docker sh
2. Try any wp command without --allow-root.
3. Smile.

[github-actions]

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