Update docker to use PHP 7.4

[kraftbj]

Eventually, this can be merged to set PHP 7.4 as the default environment for Jetpack, but while PHP 7.4 is still in development, this branch can be used for testing Jetpack to find more places for us to improve 7.4 compat.

Changes proposed in this Pull Request:

• Updates docker to use php 7.4.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• p9dueE-11V-p2

Testing instructions:

• yarn docker:clean && yarn docker:up (or docker:ngrok-up)
• Use WP and Jetpack
• Review docker/logs/php/errors.log
• Any from Jetpack? Create new issues or PRs to resolve.

Proposed changelog entry for your changes:

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: January 14, 2020.
Scheduled code freeze: January 7, 2020

Generated by 🚫 dangerJS against 451f0dd

[simison]

The best would be to have a flexible setup where we could switch PHP version "on the fly" and it would only change one image.

Prior art: #9252

[jeherve]

This seems to work well for me, it should be good to merge.

I stumbled across some notices, opened #14192.