Deployment stage failing due to heroku-php-apache2 problem

Hi everyone. As part of the project I’m building, i’ve started adding some composer dependencies. This is what my composer.json file looks like:

{
    "require": {
        "vlucas/phpdotenv": "^5.3"
    },
    "config": {
        "vendor-dir": "build/assets/php/vendor"
    }
}

Ever since adding this (it used to be an empty json object), after the build process finishes, the deployment fails with the following error:

heroku-php-apache2: command not found

I’m using webpack to build my site in a folder called build:

• webpack --mode production && composer update

and then the run command is set as heroku-php-apache2 build/. This is where the above error occurs:

heroku-php-apache2: command not found

Bit stumped on this. Is my composer.json file missing some specific information now?