Added PUBLIC_DIR argument to customise web directory

[Patabugen]

Allows users to change the public-directory name from web to anything they like by (optionally) adding a PUBLIC_PATH argument to the docker-compose file.

I've applied the change to 7.4, 8.0 and 8.1 runtimes and it should be backwards compatible without a problem.

If users have published the Dockerfile or supervisord.conf they'll need to update both if they want to use the argument.

Resolves: #4

Adds these docs to the readme:

Alternative Public Path

If your WordPress files are in a web/ directory in your repository then you're good to go - otherwise set the PUBLIC_DIR arg in docker-compose.yaml.

version: '3'
services:
    wordpress.test:
        build:
            context: ./vendor/sterner-stuff/wordpress-sail/runtimes/8.1
            dockerfile: Dockerfile
            args:
                WWWGROUP: '${WWWGROUP}'
                PUBLIC_DIR: 'public' <-- path to the directory containing wp-config.php relative to your docker-compose.yaml file
        # ...