Wordpress as Twelve-Factor. Please check https://github.com/dzuelke/wordpress-12factor for the real thing!

._wordpress.sh

mkdir wordpress
cd wordpress
git init
curl -LO https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/composer.json
curl -LO https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/.gitignore
git add composer.json .gitignore
git commit -m "composer plus wordpress prerequisites"
composer require ext-gd:* johnpbloch/wordpress wpackagist-plugin/amazon-web-services wpackagist-plugin/amazon-s3-and-cloudfront wpackagist-plugin/sendgrid-email-delivery-simplified
git add composer.json composer.lock
git commit -m "require ext-gd, wordpress and plugins"
curl -LO https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/wp-config.php
git add wp-config.php
git commit -m "wp-config with many env vars"
curl -L https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/.htaccess > wordpress/.htaccess
git add wordpress/.htaccess
git commit -m "htaccess"
curl -LO https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/Procfile
git add Procfile 
git commit -m "Heroku Procfile"
composer require wp-cli/wp-cli
curl -LO https://gist.githubusercontent.com/dzuelke/8ef648676cab07f2a177/raw/wp-cli.yml
git add composer.json composer.lock wp-cli.yml
git commit -m "WP-CLI"
heroku create
heroku addons:create jawsdb
heroku addons:create bucketeer
heroku addons:create sendgrid
heroku config:set $(curl 'https://api.wordpress.org/secret-key/1.1/salt/' | sed -E -e "s/^define\('(.+)', *'(.+)'\);$/WORDPRESS_\1=\2/" -e 's/ //g')
git push heroku master
# as an alternative to the next step, to interactively provide info (use a format like "http://example.herokuapp.com" with your app name for the URL), you can run:
# heroku run 'vendor/bin/wp core install --prompt'
heroku run 'composer wordpress-setup-core-install -- --title="Wordpress on Heroku" --admin_user=admin --admin_password=admin --admin_email=admin@example.com --url="http://example.herokuapp.com/"'
heroku run 'composer wordpress-setup-finalize'
heroku open

.gitignore

/vendor/
/wordpress/*
!/wordpress/.htaccess
!/wordpress/wp-content/themes
/wordpress/wp-content/themes/*
!/wordpress/wp-content/themes/*-child/

.htaccess

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

composer.json

{
	"repositories": [
		{
			"type": "composer",
			"url": "http://wpackagist.org"
		}
	],
	"extra": {
		"wordpress-install-dir": "wordpress",
		"installer-paths": {
			"wordpress/wp-content/mu-plugins/{$name}/": ["type:wordpress-muplugin"],
			"wordpress/wp-content/plugins/{$name}/": ["type:wordpress-plugin"],
			"wordpress/wp-content/themes/{$name}/": ["type:wordpress-theme"]
		}
	},
	"scripts": {
		"wordpress-setup-core-install-prompt": "wp core install --prompt",
		"wordpress-setup-core-install": "wp core install --skip-email",
		"wordpress-setup-clear-home": "wp option delete home || true",
		"wordpress-setup-configure-s3": "echo '{\"domain\":\"path\",\"copy-to-s3\":\"1\",\"serve-from-s3\":\"1\",\"ssl\":\"request\"}' | wp option add tantan_wordpress_s3 --format=json",
		"wordpress-setup-enable-plugins": "wp plugin activate amazon-web-services amazon-s3-and-cloudfront sendgrid-email-delivery-simplified",
		"wordpress-setup": [
			"@wordpress-setup-core-install-prompt",
			"@wordpress-setup-finalize"
		],
		"wordpress-setup-finalize": [
			"@wordpress-setup-clear-home",
			"@wordpress-setup-configure-s3",
			"@wordpress-setup-enable-plugins"
		]
	}
}

Procfile

web: vendor/bin/heroku-php-apache2 wordpress/

wp-cli.yml

path: wordpress
apache_modules: [ mod_rewrite ]
disabled_commands: [ "core config" ]

wp-config.php

<?php

$db = array_merge(['port' => 3306], parse_url(getenv('JAWSDB_URL')?:getenv('CLEARDB_DATABASE_URL')));
define('DB_NAME',     substr($db['path'], 1));
define('DB_USER',     $db['user']);
define('DB_PASSWORD', $db['pass']);
define('DB_HOST',     $db['host'].':'.$db['port']);
define('DB_CHARSET', 'utf8');
define('DB_COLLATE', '');
$table_prefix  = 'wp_';

define('AWS_ACCESS_KEY_ID', getenv('AWS_ACCESS_KEY_ID')?:getenv('BUCKETEER_AWS_ACCESS_KEY_ID'));
define('AWS_SECRET_ACCESS_KEY', getenv('AWS_SECRET_ACCESS_KEY')?:getenv('BUCKETEER_AWS_SECRET_ACCESS_KEY'));
define('AS3CF_BUCKET', getenv('S3_BUCKET')?:getenv('BUCKETEER_BUCKET_NAME'));
if(getenv('S3_REGION')) define('AS3CF_REGION', getenv('S3_REGION'));

define('SENDGRID_AUTH_METHOD', 'credentials');
define('SENDGRID_USERNAME', getenv('SENDGRID_USERNAME'));
define('SENDGRID_PASSWORD', getenv('SENDGRID_PASSWORD'));
define('SENDGRID_SEND_METHOD', 'api');

define('AUTH_KEY',         getenv('WORDPRESS_AUTH_KEY')        ?:'put your unique phrase here');
define('SECURE_AUTH_KEY',  getenv('WORDPRESS_SECURE_AUTH_KEY') ?:'put your unique phrase here');
define('LOGGED_IN_KEY',    getenv('WORDPRESS_LOGGED_IN_KEY')   ?:'put your unique phrase here');
define('NONCE_KEY',        getenv('WORDPRESS_NONCE_KEY')       ?:'put your unique phrase here');
define('AUTH_SALT',        getenv('WORDPRESS_AUTH_SALT')       ?:'put your unique phrase here');
define('SECURE_AUTH_SALT', getenv('WORDPRESS_SECURE_AUTH_SALT')?:'put your unique phrase here');
define('LOGGED_IN_SALT',   getenv('WORDPRESS_LOGGED_IN_SALT')  ?:'put your unique phrase here');
define('NONCE_SALT',       getenv('WORDPRESS_NONCE_SALT')      ?:'put your unique phrase here');

define('FORCE_SSL_ADMIN', true);
define('FORCE_SSL_LOGIN', true);
if(isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') $_SERVER['HTTPS'] = 'on';

define('WP_DEBUG', true);
define('WP_DEBUG_DISPLAY', false);
define('WP_DEBUG_LOG', false); // this is correct - we don't want errors to go to debug.log, but to the default destination

define('DISALLOW_FILE_MODS', true);

define('DISABLE_WP_CRON', in_array(getenv('DISABLE_WP_CRON'), ['true', '1', 'yes'], true) ? true : false);

if(!defined('ABSPATH')) define('ABSPATH', dirname(__FILE__) . '/wordpress/'); // should not be necessary

require_once(ABSPATH . 'wp-settings.php');

// installs using a Heroku button do not know the URL, so they use example.com as the site URL, which we need to fix
if(function_exists('get_option') && get_option('siteurl') == 'http://example.herokuapp.com') {
	update_option('siteurl', set_url_scheme($url = 'http://'.$_SERVER['HTTP_HOST']));
	header("Location: $url".$_SERVER['REQUEST_URI']);
	exit;
}

👍

I will like to know how to remove footer credit in Wordpress in a customized theme