Update Docker image to a PHP8 version

[darylldoyle]

Description of the Change

Very few projects are now using < PHP 8, and there are issues with the current snapshots pushing PHP 8 code, since the docker image is running PHP 7.3.9.

This updates the docker image to use PHP8 and adds support for ARM processors. See the notes by @dustinrue in #92, which this builds upon.

This PR modifies the Dockerfile and build routines (actions) to:

• Build the image off of the updated upstream image 10up/wp-php-fpm-dev:7.4-ubuntu 10up/wp-php-fpm-dev:8.0-ubuntu
• Provide a multi-stage build to reduce overall image size (using 10up/base-php:7.4-ubuntu 10up/base-php:8.0-ubuntu)
• Updates the Github action to build for both amd64 and arm64 (adding native, full speed support for Apple's M series of processor including M1)

Closes #94 #83 #92

How to test the Change

Changelog Entry

• Update base image and create multi-arch image compatible with arm64 and Apple M series processors

Credits

Props @dustinrue @darylldoyle

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.