Provide a Docker image

[kasparsd]

Fixes #322

Include a Docker container that can be used to run all tasks.

Projects using wp-dev-lib can now use the pre-build Docker image to run all tasks supported by this tool in an environment that includes all the required dependencies and tools:

• PHP (currently set to 7.4 but could be expanded to 7.2, 7.3 and 7.4 if needed)
• Node.js
• Composer
• WordPress development files with unit test library
• Xdebug for reporting phpunit coverage
• and some others.

To use the Docker image, add docker-compose.yml to your project:

version: '3.6'
services:
  wpdevlib:
    image: xwpco/wp-dev-lib:latest
    working_dir: /var/www/html
    volumes:
      - .:/var/www/html

and use the following command to run any composer or npm task in the container:

docker-compose run wpdevlib composer test

where composer test is the task to run, which can potentially call vendor/xwp/wp-dev-lib/scripts/pre-commit to run the dev-lib pre-commit hooks.