Add a note about lack of upstream support for PHP-8

> **Note:** Building manually requires Docker 18.09 or later with the 

> [Buildkit][] feature enabled.

> **Note:** *(Dec 2020)* Currently WP-CLI does not support PHP-8. Likely various plugins and

> themes also do not work with PHP-8. Therefore until wider support is available, supply

> a PHP-7 version with a build argument.

To build the PHP-FPM image run:

```shell

DOCKER_BUILDKIT=1 docker build -t wordpress:tag .

DOCKER_BUILDKIT=1 docker build -t wordpress:tag --build-arg php_version=7.4.13 .

```

To build the Nginx companion image, run: